T
COMETS
LONDON : PRINTED BY
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AND PARLIAMENT STREET
PL VI.
Warren DeZa Rue del
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THE GIREAT COMET OF US 61
AS SEEN BY WARREN DE LA RUE. D.C.L.. F. R.S.
^VITH HIS NEWTONIAN EQUATOREAL
OF 13 INCHES APERTURE
X
f
THE
WOELD OF COMETS
BY
AMEDEE GUILLEMIN
AOTHOK OF ' THE HEAVElfS '
TRANSLATED AND EDITED BY
JAMES GLAISHEE, F.E.S.
NUMEROUS WOODCUT ILLUSTRATIONS and CHROMOLITHOGRAPHS
LONDON
SAMPSON LOW, MAESTON, SEAELE, & EIVINGTON
CROWN BUILDINGS, 188 FLEET STREET
1877
All rights reserved
EDITOB'S PREFACE.
I HAVE great pleasure in introducing to English readers M.
GUILLEMIN'S valuable and interesting work on comets. When
rapid progress has been made in any branch of science, it is
generally very difficult for anyone, who has not been actually
concerned in the investigations in question, to obtain accurate,
information of the state of our knowledge; and for this reason
a book, such as the present, which gives an account of the new
results that we owe to very recent researches, really confers a
benefit upon many persons who, though taking a strong interest
in the subject, have necessarily been quite unable to follow its
development in the periodical publications of English and
foreio-n scientific societies. There is no work that at all
o
occupies the ground covered by that of M. GUILLEMIN ; and as
the subject is one which, always of high interest, has in the
last few years acquired great importance in consequence of
Schiaparelli's discovery of a connexion between cornets and
shooting- stars, I was anxious that it should appear in our
language.
Whenever I have thought that additional explanation was
desirable, or that the researches of the two years that have
elapsed since the publication of the original work threw further
EDITOR'S PREFACE.
light upon the subject, I have added a note of my own, such
notes being always enclosed in square parentheses [ ] ; and
although I must not be understood to endorse M. GUILLEMIN'S
o
conclusions in every case where I have not added a note, still
I may~say that there are very few of his views from which I
should feel at all inclined to dissent. Of course T have corrected
in the text all errors I have met with, which were evidently
purely accidental, and such as always will occur in the first
edition of any work. In two cases I have ventured to make
more lengthy additions of my own — these relate to Coggia's
comet, which had only just left us when M. GUILLEMIN'S work
was published, and the connexion of comets and shooting- stars.
Some remarks will also be found in the note which follows the
catalogue of comets at the end of the book.
In conclusion, I must express my thanks to Dr. WARREN DE
LA RUE, F.R.S., for having very kindly placed at my disposal
copies of his beautiful drawings of the great comet of 1861,
which add greatly to the value of the work.
JAMES GLAISHER.
BLACKHEATH, S.E. : Nov. 15, 1876.
PREFACE.
THE UNIVEKSE is formed of an infinity of worlds similar to
our own. The thousands of stars which meet our gaze in the
azure vault of the heavens when we contemplate it with the
naked eye, and which may be reckoned by hundreds of millions
when we explore its depths by the aid of the telescope, are
suns. These foci of light, these sources of heat, and incon-
testably of life, are not ioolated; they are distributed into
groups or clusters ; sometimes by twos or threes, sometimes
by hundreds, sometimes by myriads ; the clouds of vaporous
light called nebula? are for the most part thus constituted.
Isolated or in groups, the stars seem to us immovable, so
prodigious is the distance by which they are separated from
the earth and from our sun. They move nevertheless ; and
amongst those whose velocities have as yet been measured
may be reckoned some which are moving ten times and even
fifty times quicker than a cannon-ball when it leaves the
cannon. Movement is, therefore, the most universal law of the
stars.
In like manner our sun moves through space and compels
the earth to follow. He bears along with him, in this voyage
through the boundless ether, the globes which form his cortege
and gravitate about his enormous mass. During the thousands
of years that man has been a witness — an unconscipus witness,
PREFACE.
it is true— of this circumnavigation of the universe, he has
seen no change in the aspect of the surrounding worlds; the
sidereal shores of the ocean in which this fleet of more than
a hundred celestial bodies pursues its way preserves to all
appearance its unchanging front. The immensity of the
sidereal distances, it is well known, is the sole cause of this
apparent immobility.
The solar world is, therefore, separated from all other worlds
by unfathomable abysses ; the sun is as it were isolated, lost in
a corner of space, far from the millions of stars with which
nevertheless it forms a system. Member of an immense asso-
ciation, integral molecule of the most vast, to all appearance, of
the nebula?, the Milky Way, the central star of our group
seems to have no other mode of communicating with its co-
associates than by the reciprocal exchange of undulations, that
is to say, by the exchange of light and heat. Like disciplined
and devoted soldiers, the earth and the planets march in com-
pany with the sun, effecting with marvellous regularity their
nearly circular revolutions around their common focus, and
never deviating from the limits imposed upon them by the
law of gravitation.
They remain, therefore, isolated like the sun, separated
from other sidereal systems by distances so enormous that the
mind is powerless to conceive of them.
A relation, however, exists between our system and these
systems, as we have just mentioned: the sun is a star of the
Milky Way. But, we may ask, has the solar world no closer
and more direct connexion with the rest of the visible universe ?
The movement of translation by which it is animated
proves at least that in some quarter of the heavens there is
either an unknown celestial body, or a system of celestial
bodies, around which gravitation causes the group to describe
an orbit of undetermined period. And this movement of the
PREFACE
whole results from the concurrent action of all the stars in
the universe. The force of gravitation is, therefore, a common
bond of union between our world and all others.
Is it, therefore, steadily advancing to some celestial archi-
pelago which it will finally attain in a few millions of years ?
Are, then, future generations destined to see other suns, from
other points of view ? These are questions whose solution may
be considered inaccessible to us.
But, amongst the stars of which the solar system is com-
posed, are there not some less immutably attached than the
planets and the earth to the focus of their movement? Are
there not some which depart to a greater distance from their
focus, and which, like messengers detached from the group,
carry to neighbouring world news of our own ?
Such a hypothesis is not without foundation.
Astronomers, in fact, have for the last two centuries
studied the movement of certain celestial bodies, which come
to us and gravitate about the sun, but which, after having, so to
speak, saluted on their way the ruler of the planets, return and
plunge again to immeasurable distances in the depths of ether.
A small number of these stars, retained by the solar power,
diverted from their path by the influence of some of the larger
planets, have remained tributaries of the group of which they
now form an integral part.
These singular stars, long disowned, are COMETS.
I have said long disowned. Comets, indeed, have only
been considered during the last two centuries as properly
belonging to the family of the stars : before Newton's time
they were regarded even by astronomers as transient meteors,
whose appearance, disappearance, and movements were subject
to no law. For the ancients, and the world in general during
the Middle Ages, and even during the Renaissance, they were
objects of fear, miraculous apparitions, signs the- precursors
PREFACE.
of terrible calamities, flaming symbols of the Divine anger.
For the savants of former times comets were the monsters of
the sky.
Two centuries of scientific progress exhibit these calumniated
stars in a very different light. Thanks to Newton's discovery
of gravitation, and to the united efforts of mathematicians and
observers, their courses have been reduced to the same laws
as those which govern the movements of the planets. These
vagabonds of the sky have testified, some by their regular motion,
others by their return at the predicted dates, their submission to
the laws of celestial mechanics. Their very deviations have been
recognised as the legitimate consequence of foreign influences.
For some years more has been done : an endeavour has been
made, and with some success, to penetrate the mystery of their
organisation, and to discover the physical and chemical nature
of their light; but more especially the part which they perform
in the solar system and in the universe itself is beginning to
be viewed in its proper light.
Comets, as Laplace had foreseen, are of different origin to
the planets. The eccentricity of their orbits, the inclinations of
the planes in which they move, their course, sometimes direct,
sometimes retrograde, mark a profound distinction between
them and the planets. Their interior structure, the nebulous
appearance of nearly all, the rapid changes observed both in
their nuclei and atmospheres, remove them no less from the
permanent and globular figure, either solid or liquid, of the
majority of the planets. Some comets move in infinite orbits.
They are, therefore, strangers to our world, which they visit
on their journey. Those which are periodical have most fre-
quently such lengthened orbits that, after voyages the durations
of which are measured by thousands of centuries, they are cast
adrift far from the sun, far from the directing focus of move-
ment. Is it certain that they will rejoin him on their return,
PREFACE.
and that these wanderers will not be, in the end, stars lost to
our world?
In any case, they come to us out of the depths of infinite
space. And if the views of M. Hoek, a Dutch savant, are
well founded, it is not singly but in groups that these nebu-
losities— let us say nebulas, since the structure of comets
appears analogous to that of the nebulas properly so called
— quit the sidereal depths and penetrate to the heart of our
system. Here, then, is the material bond, the real connexion
establishing a direct and uninterrupted communication between
the solar world and the millions, the thousands of millions of
stars which constitute the splendour of the heavens.
But the physical nature of these frail messengers of space
is such, that they cannot without injury pass through the
regions traversed by the planets and the sun. So great is the
' swell ' engendered by the» motions of these massive stars,
that comets when navigating in these agitated deeps of the
ethereal sea are there subjected to considerable damage ;
sometimes they are shattered and broken into fragments ; fre-
quently they leave behind them debris which follows in their
wake. The interplanetary spaces are in this way strewn with
the particles which the planets meet with in their periodical
course, and which cause our nights to be illuminated mo-
mentarily with brilliant trains of light.
The SHOOTING-STARS are due to these rencontres.
Ten years have hardly elapsed since Schiaparelli, a learned
Italian astronomer, by a happy idea connected meteoric with
cometary astronomy. If this bold theory, involving, if not the
identity, at least the community of origin between meteors and
comets, be true, how important do the latter suddenly become
in the economy of the universe ! Travelling from world to
world, scattering upon their route in the neighbourhood of the
permanent stars of each system the dust of the elements of
XI
PREFACE.
which they are composed, may it not be that they modify in the
course of time the structure of these stars themselves ?
spectral analysis does not err, the matter of comets is chiefly
composed of carbon combined with some other element, such
as hydrogen. Here, then, are comets, shedding these sub-
stances so important to vegetable and animal life, first in the
interplanetary spaces, and then, by the fall and combustion of
meteors, in the atmospheres of the planets ; thereby, perhaps,
maintaining life upon them.
The unformed portions of matter distributed in immense
masses in certain unresolvable nebulas, and successively de-
tached in separate globules, would continue to describe hyper-
bola? or other curves of endless branches, on their passage from
star to star, and from world to world : these masses of vapour
would be presented to us under the form of comets or long
trains of vapour.
Comets, therefore, which during the reign of ignorance and
superstition were looked upon as scourges, are, more probably,
not only inoffensive stars but, perhaps, even beneficent regene-
rators of life in more advanced worlds.
These views, it is true, are only hypotheses: we know so
little of that department of astronomical science which a
great writer of our time has called by anticipation Celestial
Organism, in opposition to Celestial Mechanics, of which our
knowledge is now so far advanced. But they suffice to show
what interest, scientific and philosophic, attaches to the subject
of this work. Comets up to the present time have furnished
only one chapter, and that the briefest, to the science of the
heavens. The preceding considerations will have sufficed to
show that they perform a part of great importance in the
universe, and that their history merits more ample develop-
ment than has been accorded to it in most treatises.
Besides, apart from the scientific interest of the subject, an
Xll
PREFACE.
historic interest attaches to it. Considered from this point of
view, comets would furnish matter for an interesting work.
In the volume before the reader will be found a few chapters
devoted to a brief history of what may be called Cometary
Astrology. It forms a necessary introduction to the purely
astronomical portion ; and, were it omitted, it would be difficult
to understand how the world has passed from the most extra-
vagant prejudices to the calm and reassuring conceptions of
contemporary science.
In ancient Greece, in heroic times, comets, as indeed all ce-
lestial phenomena, excited only graceful ideas. Take, for ex-
ample Homer: it is Minerva and Apollo, the two brilliant deities
of Olympus, who thus manifest themselves to mortals. Later
on, they became fatal presages. The Romans, more austere,
had already interpreted them as signs of fatal augury, forerun-
ners of calamity. In the Middle Ages the ideas connected
with the m continued to increase in gloom : comets were then
stars only of misfortune, ruin, and death. The terrible and
grandiose idea of the end of the world, so universal at that
period of darkness, predominated over all and set its seal on
all. At last, with the revival of learning, scientific observation
slowly dissipated these prejudices. In the eighteenth century
the light of a free interpretation of nature resumed its empire :
comets were spoken of without awe, and these stars, but lately
so formidable, became even a theme for satire.
Contemporary science, more profound, restores to comets
their majesty and importance, but it also despoils these appa-
ritions for ever of all significance derived from idle superstition
and terror.
CONTENTS.
CHAPTER I.
BELIEFS AND SUPERSTITIONS RELATIVE TO COMETS.
SECTION I.
COMETS CONSIDERED AS PRESAGES.
PAGE
Comets have been considered in all times and in all countries as signs, precursors
of fatal events — Antiquity and universality of this belief ; its probable origin
— Opinion of Seneca ; habitual and regular phenomena fail to attract the
attention of the multitude ; meteors and comets, on the contrary, make a
profound impression — The moderns in this respect resemble the ancients
contemporary with Seneca — The incorruptible heavens of the ancients, in
contradistinction to the sublunary or atmospheric regions ; stars and meteors
— Inevitable confusion of certain celestial or cosmical phenomena with
atmospheric meteors ........ 3
SECTION II.
COMETS IN GREEK AND ROMAN ANTIQUITY.
The apparition of a comet or a bolide is a warning from the gods : the Iliad and
the ^Eneid — Supposed physical influences of comets ; Earthquakes in Achaia ;
submersion of Helice and Bura ; comet of the year 371 — Comets, presages of
happy augury ; Csesar transported to the heavens under the form of a comet ;
popular credulity turned to account; opinion of Bayle — Pliny, Virgil, Tacitus,
Seneca — The comet of the year 79 and the Emperor Vespasian — Comet of the
year 400 and the siege of Constantinople .....
CONTENTS.
SECTION III.
THE COMETS OF THE MIDDLE AGES. ^^
™ters-Halley's comet and the Turks; origin of the Angelas <U M«k-
The clet onm and the conquest of England by the Normans ; apostrophe ^
to the comet by a monk of Malmesbury
SECTION IV.
COMETS FROM THE RENAISSANCE TO THE PRESENT DAY.
Slow improvement in the beliefs relative to comets-Bayle's remarks upon the
comet of 1680-Passage fromMadame de Sevigne's letter referring to this comet
and the last hours of Mazarin-In the eighteenth century belief in the super-
natural exchanged for belief in the physical influence of comets-Remains oi
cometary superstitions in the nineteenth century— The comet of 1812 and the
Russian campaign ; Napoleon I. and the comet of 1769 ; the great comet of
1861 in Italy ...•••••'
CHAPTER II.
COMETARY ASTRONOMY TIP TO THE TIME OF NEWTON.
SECTION I.
COMETS AND THE ASTRONOMERS OF EGYPT AND CHALDEA.
Had the Egyptians and Chaldeans any positive knowledge concerning comets ? —
Apollonius of Myndus ; the Pythagoreans considered comets to be true stars
According to Aristotle they are transient meteors ; fatal influence of the
authority of this great philosopher upon the development of Cometary As-
tronomy .......-•*"*'
SECTION II.
COMETARY ASTRONOMY IN THE TIME OF SENECA.
Book vii. of Seneca's Qu&stiones Ncdurales relates to comets — Seneca defends in
it the system of Apollonius of Myndus ; he puts forth just views concerning
the nature of comets and their movements — His predictions respecting future
discoveries in regard to comets — The astronomers of the future . . 42
CONTENTS.
SECTION III.
COMETS DURING THE RENAISSANCE AND UP TO THE TIME OF
NEWTON AND HALLEY.
PAGK
Apian observes that the tails of comets are invariably directed from the sun —
Observations of Tycho Brahe ; his views and hypotheses concerning the
nature of comets — Kepler regards them as transient meteors, moving in
straight lines through space — Galileo shares the opinion of Kepler — Systems
of Cassini and Hevelius . . . . . . .47
SECTION IV.
NEWTON DISCOVERS THE TRUE NATURE OF COMETARY ORBITS.
Newton's Principia and the theory of universal gravitation — Why Kepler did not
apply to comets the laws of the planetary movements — Newton discovers the
true system of cometary orbits — Halley and the comet of 1682 ; prediction of
its return .... 52
CHAPTER III.
THE MOTIONS AND ORBITS OF COMETS.
SECTION I.
COMETS PARTICIPATE IN THE DIURNAL MOTION . . 59
SECTION II.
MOTIONS OF COMETS.
Distinction between comets, nebulse, and temporary stars— Comets, in their
motions, are subject to stationary periods and retrogressions — The apparent
complications arise, as in the case of the planets, from the simultaneous move-
ment of these bodies and the earth . . . . . .61
SECTION III.
IRREGULARITIES IN THE MOTIONS OF COMETS.
Comets appear in all regions of the heavens — Effects of parallax — Apparent
motion of a comet, in opposition and in perihelion, moving in a direction
opposite to the earth — Hypothetical comet of Lacaille; calculations of
Lacaille and Olbers concerning the maximum relative movement of this
hypothetical comet and the earth . . . . . .65
xvii a 2
CONTENTS.
SECTION IV.
THE ORBITS OF COMETS.
PAGE
Kepler's Laws ; ellipses described around the sun ; the law of areas— Gravitation,
or weight, the force that maintains the planets in their orbits— The law of
universal gravitation confirmed by the planetary perturbations— Circular,
elliptic, and parabolic velocity explained ; the nature of an orbit depends
upon this velocity— Parabolic elements of a cometary orbit . . .69
SECTION V.
THE ORBITS OF COMETS COMPARED WITH THE ORBITS OF THE PLANETS.
Differences of inclination, eccentricity, and direction of motion . . .83
SECTION VI.
DETERMINATION OF THE PARABOLIC ORBIT OF A COMET.
Three observations are necessary for the calculation of a parabolic orbit — Cometary
ephemerides ; what is meant by an ephemeris ; control afforded by the
ulterior observations — Elements of an elliptic orbit — Can the apparition or
return of a comet be predicted ? — State of the question — Refutation by Arago
of a current prejudice ........ 87
CHAPTER IV.
PEEIODICAL COMETS.
SECTION I.
COMETS WHOSE RETURN HAS BEEN OBSERVED.
How to discover the periodicity of an observed comet and predict its return —
First method : comparison of the elements of the orbit with those of comets
that have been catalogued— Resemblance or identity of these elements : pre-
sumed period deduced from it— Second method : direct calculation of elliptic
elements — Third method 95
CONTENTS.
SECTION II.
HALLEY'S COMET.
PAGE
Discovery of the identity of the comets of 1682, 1607, and 1631 ; Halley
announces the next return for the year 1758 — Olairaut undertakes the calcu-
lation of the disturbing influence exercised by Jupiter and Saturn upon the
comet of 1682 ; collaboration of Lalande and Mdlle. Hortense Lepaute — The
return of the comet to its perihelion is fixed for the middle of April 1759 ;
the comet returns on the 13th of March — Return of Halley's comet in 1835 ;
calculation of the perturbations by Damoiseau and Ponte"coulant ; progress of
theory — The comet will return to its perihelion in May 1910 . . 100
SECTION III.
ENCKE'S COMET ; OB, THE SHORT PERIOD COMET.
iDiscovery of the identity and periodicity of the comets of 1818, 1805, 1795, and
1786 ; Arago and Olbers — Encke calculates the ellipse described by the
comet — Dates of twenty returns up to 1873— Successive diminution of the
period of Encke's comet •••.... 109
SECTION IV.
BIELA'S OR GAMBART'S COMET.
History of its discovery ; its identification with the comet of 1805 — Calculation
of its elliptic elements by Gambart — Apparitions previous to 1826 — Pecu-
liarities in the apparitions of 1832, 1846, and 1872 . . . .113
SECTION V.
FATE'S COMET.
First comet whose periodicity, without comparison with previous dates, has been
determined by calculation and verified by observation — M. Le Verrier demon-
strates that it has nothing in common with the comet of Lexell — Slight
eccentricity of Faye's comet and great perihelion distance — Dates of its
return — Perturbations in the movements of Faye's comet inexplicable by
gravitation alone : a problem to be solved . . . . .116
SECTION VI.
BRORSEN'S COMET.
Discovery of the comet of five years and a half period by Brorsen in 1846 — Its
supposed identity with the comet of 1532 gives reason to suspect elliptic
elements; calculation of these elements — Returns of the comet in 1851, 1868,
and 1873 .... .... 119
"
CONTENTS.
SECTION VII.
D'ARREST'S COMET.
PAGE
Discovery of the comet and of its periodicity by D' Arrest— Return predicted by
M. Yvon Villarceau for 1867 ; verification to within half a day— Importance
of the perturbations caused by Jupiter— Research of MM. Yvon Villarceau
and Leveau— Return of the comet in September 1870 . .122
SECTION VIII.
TUTTLE'S COMET.
The period of Tattle's comet is intermediate to that of Halley's comet and those
of other periodical comets that have returned — Very elongated orbit of the
comet of 13f years period — Previous observation in 1790; five passages not
since observed — Next return in September 1885 .... 124
SECTION IX.
WIXNECKE'S PERIODICAL COMET.
Discovery of the periodicity of the third coinet of 1819 ; calculation of its elliptic
elements by Encke — Discovery of Winnecke's comet in 1858 ; its identity
with the comet discovered by Pons — Return of the star to its perihelion in
1869 ; probable date of its next return in 1875 .... 126
SECTION X.
TEMPEL'S SHORT PKEIOD COMET.
Calculation of the elliptic elements of the second comet of 1867, discovered by
Tempel— Perturbations due to Jupiter, and consequent delay in the return of
the comet to its perihelion in 1873— Remarkable agreement of observation
and calculation ... 128
CONTENTS.
CHAPTER V.
PERIODICAL COMETS.
SECTION I.
COMETS WHOSE RETURN HAS NOT BEEN VERIFIED BY OBSERVATION.
PACK
Periodical comets which have not been seen again; long periods; circumstances
unfavourable to observation ; motions possibly disturbed by perturbations —
Elliptic orbits determined by calculation — Uncertainty of return under these
different hypotheses ........ 131
SECTION II.
INTERIOR COMETS, OR COMETS OF SHORT PERIOD, THAT HAVE NOT YET
RETURNED.
Comets lost or strayed : the comet of 1743 ; the comet of Lexell, or 1770; per-
turbations caused by Jupiter ; in 1767 the action of Jupiter shortens the
period, and in 1779 produces an opposite effect — Comet of De Vico; short
period comets of 1783, 1846, and 1873 . . . . .133
SECTION III.
COMETS OF MEAN PERIOD.
Periodical comets exterior to the solar system ; the type of this class is Halley's
comet, which is the only comet of mean period whose return has been verified
by observation — Enumeration of comets with periods between 69 and 200
years — Periods ; aphelion and perihelion distances . . . .141
SECTION IV.
COMETS OF LONG PERIOD.
Periodical comets exterior to the known limits of the solar system — Distance to
which the comet of longest calculated period recedes from the sun — The so-
called comet of Charles V. ; its apparitions in 1264 and 1556; its return
predicted for the middle of the nineteenth century, between 1848 and 1860 —
Calculation of the perturbations ; another comet lost or strayed — The great
comet of 1680 ; the Deluge and the end of the world — Magnificent comets of
1811, 1825, and 1843 144
CONTENTS.
CHAPTER VI.
TBS WORLD OF COMETS AND COMETARY SYSTEMS.
SECTION I.
THE NUMBER OF COMETS.
Arago-Calculation of the probable number of comets from the actual data ; ^
Kepler's remark verified
SECTION II.
COMETS WITH HYPERBOLIC ORBITS.
Do all comets belong to the solar system ?-Orbits which are clearly hyperbolic
—Opinion of Laplace with regard to the rarity of hyperbolic comets— Are
there any comete which really describe parabolas ?-First glance at the origin
. Io7
of comets
SECTION III.
REMARKS ON THE ORIGIN OF COMETS.
Have all the known comets of the solar world always belonged to it ?— Probable
modification of their original orbits through the planetary perturbations-
Cause of the gradual diminution of the periods of certain cornets . . 171
SECTION IV.
SYSTEMS OF COMETS.
Comets which have or seem to have a common origin — Double comets — Systems
of comets according to M. Hoek — Distribution of aphelia over the celestial
vault ; region of the heavens particularly rich in aphelia . . 174
SECTION V.
COMETARY STATISTICS.
Comparison of the elements of cometary orbits — Eccentricities ; numbers of
elliptic, parabolic, and hyperbolic comets— Distribution of comets according
to their nodes and perihelion distances — Equality of the numbers of direct
and retrograde orbits ........ 182
CONTEXTS.
CHAPTER VII.
PHYSICAL AND CHEMICAL CONSTITUTION OF COMETS.
SECTION I.
COMETS PHYSICALLY CONSIDERED.
PAGE
The physical or chemical constitution of a celestial body ; nature of the question
involved •, explained by reference to the earth — A cometary problem . 193
SECTION II.
COMETARY NUCLEI, TAILS, AND COM.S.
Comae and tails — Classification of the ancients according to apparent external
form ; the twelve kinds of comets described by Pliny— The ' Guest-star ' of
the Chinese — Modern definitions : nucleus, nebulosity or atmosphere ; tails . 196
SECTION III.
COMETS DEVOID OF NUCLEUS AND TAIL.
Gradual condensation of nebulous matter at the centre — Imperceptible transition
from comets without apparent tails to the immense luminous trains of great
historic comets ........ 201
SECTION IV.
DIRECTION OF THE TAILS OF COMETS.
Direction of the tail opposite to the sun ; discovered by Apian ; the Chinese
astronomers were acquainted with this law — Deviations in some comets
— Variable aspect of the tail according to the relative positions of the comet,
the earth, and the sun ....... 206
SECTION V.
NUMBER OF TAILS.
Double tails of comets; comets of 1823, 1850, and 1851— Tails multiple, fan-
shaped, rectilinear, curved — Variable number of tails belonging to the same
comet ; comets of Donati, of 1861 and of Ch^seaux . . . 209
PAGE
CONTENTS.
SECTION VI.
DIFFERENT FORMS OF TAILS.
Elementary forms of tails-Rectilinear tails, divergent or convergent in
fipect of the head of the comet-Curved tails; comets of 1811 and 176£
—Whimsical form of cometary appendages according to ancient o
vations •
SECTION VII.
LENGTH OF TAILS.
Apparent and real dimensions of the largest tails on record— Formation and de-
velopment of cometary appendages ; their disappearance— Variations of
length in the tail of Halley's comet at its different apparitions— Great comet
of 1858, or comet of Donati . • 221
SECTION VIII.
FORMATION AND DEVELOPMENT OF TAILS.
Variations of length in the tail of Halley's comet at its different apparitions —
Similar phenomena exhibited by Donati's comet in 1858 — Does the maximum
development of the tail always coincide with the perihelion passage of the
comet? ......... 224
SECTION IX.
BRILLIANCY OF COMETS.
Estimations of the apparent dimensions or brilliancy of comets— Ancient comets
said to be brighter than the sun — Comets visible to the naked eye and comets
seen at noonday ; great comets of 1744 and 1843 .... 232
SECTION X.
DIMENSIONS OF NUCLEI AND TAILS.
Real dimensions of the nuclei and atmospheres of various comets — Uncertainty
of these elements ; variations of the nucleus of Donati's comet — Observations
of Ilevelius upon the variations of the comet of 1652 — Do cometary
nebulosities diminish in size when their distance from the sun decreases ?
— Encke's comet considered in regard to this question at its apparitions in
1828 and 1838 238
xxiv
CONTENTS.
CHAPTER VIII.
PHYSICAL TRANSFORMATIONS OF COMETS.
SECTION I.
AIGRETTES — LUMINOUS SECTORS — NUCLEAL EMISSIONS.
PAGE
Predominance of atmosphere in comets — Luminous sectors; emission of vaporous
envelopes from the nucleus in the comets of 1835, 1858, 1860, and 1861 —
Formation of envelopes in Donati's comet ; progressive diminution of the
velocity of expansion in emissions from the nucleus .... 247
SECTION II.
OSCILLATIONS OF LUMINOUS SECTORS : COMET OF 1862.
M. Chacornac's observations upon the comet of 1862 — Formation of luminous
sectors emanating from the nucleus — Oscillation of aigrettes, and flowing
back of the nucleal matter ....... 254
SECTION III.
DUPLICATION OF BIELA'S COMET.
First signs of the doubling of Biela's comet, in the month of January 1846 —
Observations of the twin comets in America and Europe— Gradual separation
and approach of the fragments — The two comets return and are observed in
1852 ; their distances found to have increased — Elements of the orbits of the
two comets . . . . . . . . . 258
SECTION IV.
DOUBLE COMETS MENTIONED IN HISTORY.
Is there any example in history of the division of a comet into several parts ? —
The comet of B.C. 371 — Ephorus, Seneca and Pingre — Similar observations in
Europe and China — The Olinda double comet, observed in Brazil, in 1860, by
M. Liais. . 268
CONTENTS.
CHAPTER IX.
MASS AND DENSITY OF COMETS.
SECTION I.
FIRST DETERMINATION OF THE MASSES OF COMETS.
PAOK
Lexell's comet and the calculations of Laplace— The smallness of coraetary
masses deduced from the fact that comets exercise no disturbing influence
upon the earth, the planets, or their satellites .... 277
SECTION II.
METHOD OF ESTIMATING THE MASSES OF COMETS BY OPTICAL
CONSIDERATIONS.
The masses of Encke's comet and the comet of Taurus determined by M.
Bahinet — Objections to this method of determination . . . 281
SECTION III.
THIRD METHOD OF DETERMINING THE MASSES OF COMETS.
Theory of the formation and development of cometary atmospheres under the
influence of gravitation and a repulsive force — Calculations of M. Edouard
Roche — Masses of the comets of Donati and Encke as determined by this
method ..... 286
CHAPTER X.
THE LIGHT OF COMETS.
SECTION I.
INTEREST ATTACHING TO THE PHYSICAL STUDY OP COMETARY
LIGHT .... 291
xxvi
CONTENTS.
SECTION II.
TRANSPARENCY OF NUCLEI, ATMOSPHERES, AND TAILS-
PAGE
Visibility of stars through the atmospheres and tails of comets ; ancient and
modern observations upon this point — Are the nuclei of comets opaque, or
transparent like the atmospheres and tails ? — Reported eclipses of the sun and
moon produced by comets ....... 293
SECTION III.
COLOUR OF COMETARY LIGHT.
Different colours of the heads and tails of comets — Examples of colour taken
from the observations of the ancients : red, blood -red, and yellow comets —
Difference of colour between the nucleus and the nebulosity — Blue comets —
The diversity of colour exhibited by comets is doubtless connected with
cometary physics, and with the temperature and chemical nature of come-
tary matter ......... 299
SECTION IV.
SUDDEN CHANGES OF BRILLIANCY IN THE LIGHT OF COMETARY TAILS.
Rapid undulations occasionally observed in the light of cometary tails ; obser-
vations of Kepler, Hevelius, Cysatus, and Pingre ; comets of 1607, 1618,
1 652, 1661, and 1769— Undulations in the tails of the comets of 1843 and 1860 ;
do these undulations arise from a cause peculiar to the comet itself, or do
they depend upon the state of the atmosphere ? — Objection made by Gibers
to the first of these hypotheses; refutation by M. Liais . . . 305
SECTION V.
DO COMETS SHINE BY THEIR OWN OR BY REFLECTED LIGHT?
Do the nuclei of comets exhibit phases ? — Polarisation of cometary light — Ex-
periments of Arago and of several contemporary astronomers — The light of
nebulosities and atmospheres is partly light reflected from the sun . . 309
SECTION VI.
SPECTRAL ANALYSIS OF THE LIGHT OF COMETS.
Researches of Huggins, Secchi, Wolf, and Rayet — Spectra of different comets :
bright bands upon a continuous luminous ground — Analysis of the light of
Coggia'a comet in 1874 — Chemical composition of different nuclei and nebu-
losities ......... 315
xxvii
CONTENTS,
THE COMET OF 1874, OR COCGIA'S COMET.
PAGE
Of the five comets of 1874 the third, or comet of Coggia, was alone visible to
the naked eye— Telescopic aspect and spectrum of the comet during the early
part of its apparition, according to Messrs. Wolf and Rayet-Observations of
Secchi, Bredichin, Tacchini, and Wright; polarisation of the light of the
nucleus and tail— Transformations in the head of the comet between the 10th
of June and the 14th of July, according to Messrs. Rayet and Wolf . 328
ON COGGIA'S COMET (III., 1874).
[ADDITION BY THE EDITOR.] . 342
CHAPTER XL
THEORY OF COMETARY PHENOMENA.
SECTION I.
WHAT IS A CO M E T ?
Complexity and extent of the question — The law of gravitation suffices to
explain the movements of cornets — Lacunae in the theory ; acceleration of
the motion of the comets of Encke and Faye — Origin of comets ; their
systems — Questions relative to their physical and chemical constitution —
Form of atmospheres ; birth and development of tails . . . 357
SECTION II.
CARDAN'S HYPOTHESIS.
Cometary tails considered as effects of optical refraction — Objections made by
Newton and Gregory— New theory of Gergonne ; ideas of Saigey on the
subject of planetary tails— Difficulties and lacunce in this theory . . 361
SECTION III.
THEORY OP THE IMPULSION OF THE SOLAR RAYS.
Ideas of Kepler concerning the formation of tails— Galileo, Hooke, and Euler—
Hypothesis of Kepler formulated by Laplace— Where does the impulsion
come from in the theory of undulations ? 365
xxviii
CONTENTS.
SECTION IV.
HYPOTHESIS OF AN APPARENT REPULSION.
PAGE
Views of Newton on the formation of the tails of comets — Action of heat and
rarefaction of the cometary matter — The ethereal medium, losing its specific
weight, rises opposite the sun, and carries with it the matter of the tail —
Objections which have been made to the hypothesis of a resisting and pon-
derable medium ........ 369
SECTION V.
THEORY OF OLBERS AND BESSEL.
Hypothesis of an electric or magnetic action in the formation of tails — Repulsive
action of the sun upon the cometary matter, and of the nucleus upon the
nebulosity — Views of Sir John Herschel and M. Liais — Theory of Bessel —
Oscillations of luminous sectors — Magnetic polar force . . . 372
SECTION VI.
THEORY OF COMETARY PHENOMENA.
Researches of M. E. Roche upon the form and equilibrium of the atmospheres of
celestial bodies under the combined influence of gravitation, solar heat, and a
repulsive force — Figure of equilibrium of a solid mass submitted to gravi-
tation and the heat of the sun — Comets should have two opposite tails —
Completion of the theory of cometary tides by the admission of a repulsive
force, real or apparent — Accordance of the theory so completed with ob-
servation ......... 380
SECTION VII.
THK REPULSIVE FORCE A REAL PHYSICAL FORCE.
Theory of M. Faye — Rigorous definition of the repulsion inherent in the solar
rays — Its intensity varies with the surfaces of the two bodies ; it decreases
inversely as the square of the distance — It is not propagated instantaneously —
Discussion and accordance of the facts — Experiments in support of a repulsive
force . . . .389
SECTION VIII.
THEORY OF THE ACTINIC ACTION OF THE SOLAR RAYS.
Experiments and hypotheses of Tyndall — Originality of his theory ; objections
and omissions — Is this theory incompatible with that of a repulsive force ? . 399
CONTENTS.
SECTION IX.
COMETS AND THE RESISTANCE OF THE ETHER.
PAGE
Accelerated motion of Encke's comet; its periods continually diminish— It
describes a spiral, and. will ultimately fall into the sun-Hypothesis of a re-
sisting medium; how does the resistance of a medium increase the rapidity of
motion ?— The nature of this supposed medium, according to Arago, Encke,
and Plana— Objections of M. Faye ; the acceleration of motion explained by
the tangential component of the repulsive force . • 406
CHAPTER XII.
COMETS AND SHOOTING STAES.
SECTION I.
WHAT IS A COMET ?
The ancients were unacquainted with the physical nature of comets — False ideas
entertained by astronomers of the eighteenth century respecting the physical
constitution of comets ; comets regarded by them as globes, nearly similar to
the planetary spheroids — Views of Laplace upon comets, compared by him to
nebulae — Contemporary astronomers have confirmed these views and rectified
the errors of the ancient hypotheses — Desideratum of science ; the rencontre
of the earth with a comet or the fragment of a comet . . . 417
SECTION II.
IS THE MATTER OP COMETS DISPERSED IN THE INTERPLANETARY
SPACES? .... 422
SECTION III.
COMETS AND SWARMS OF SHOOTING STARS.
Periodicity of the meteor-swarms ; radiant points ; number of swarms recognised
at the present day — Periodical maxima and minima in certain meteoric
currents ; thirty years period of the November swarm— Parabolic velocity of
shooting stars ; the swarms of shooting stars come from the sidereal depths . 425
CONTENTS.
SECTION IV.
COMMON ORIGIN OF SHOOTING STABS AND COMETS.
PAGH
Transformation of a nebula which has entered into the sphere of the sun's
attraction ; continuous parabolic rings of nebulous matter — Similarity between
the elements of the orbits of meteor streams and cometary orbits — The
August stream ; identity of the Leonides and the comet of 1862 — Identity
of the Perseids and the comet of 1866 (Tempel) — The shooting stars of April
20 and the comet of 1861 — Biela's comet and the December stream — Did the
earth encounter Biela's comet on November 27, 1872 ? 430
ON THE CONNEXION BETWEEN COMETS AND METEORS.
BY THE EDITOR . 436
CHAPTER XIII.
COMETS AND THE EARTH.
SECTION I.
COMETS WHICH HAVE APPROACHED NEAREST TO THE EARTH.
The memoir of Lalande and the panic of the year 1773 — Letter of Voltaire upon
the comet — Announcement in the Gazette de France and the Memoirs of
Bachaumont — Catalogue given by Lalande of comets which up to that time
had approached nearest to our globe ..... 455
SECTION II.
COMETS AND THE END OF THE WORLD.
Prediction of 1816 ; the end of the world announced for July 18 — Article in the
Journal des Dtbats — The comet of 1832 ; its rencontre with the orbit of the
earth — Notice by Arago in the Annuaire du Bureau des Longitudes — Proba-
bility of a rencontre between the comet and the earth — The end of the world
in 1857 and the comet of Charles V. . . . . . 460
CONTENTS.
SECTION III.
MECHANICAL AND PHYSICAL EFFECTS OF A COLLISION WITH A COMET.
PAGE
Opinions entertained by astronomers of the last century : Gregory, Maupertuis,
Lambert— Calculations of Lalande ; comets move too rapidly in the vicinity
of the earth for the effects of their attraction to come into play— Opinion of
Laplace — The collision of a comet with the earth ; its effect according to the
mechanical theory of heat ....... 467
SECTION IV.
CONSEQUENCES OF A COLLISION BETWEEN A COMET AND THE EARTH
ACCORDING TO THE MECHANICAL THEORY OF HEAT . . . 477
SECTION V.
THE COMET OF 1680, THE DELUGE, AND THE END OF THE WORLD.
Ancient apparitions of the comet of 1680, on the hypothesis of a revolution of
575 years — Their coincidence with famous events — Whiston's theory of the
earth : our globe is an ancient comet, whose movements and constitution
have been modified by comets — The catastrophe of the Deluge caused by the
eighth anterior apparition of the comet of 1680 — Final catastrophe : burning
of the earth — Future return of our globe to the condition of a comet . 480
SECTION VI.
PASSAGE OF THE EARTH THROUGH THE TAIL OF A COMET IN 1861.
Possibility of our globe passing through the tail of a comet — Has such an event
ever taken place ?— The great comet of 1861— Relative positions of the earth
and one of the two tails of that comet— Memoir of M. Liais and the observa-
vations of Mr. Hind . . 486
xxxu
CONTENTS.
CHAPTER XIV.
PHYSICAL INFLUENCES OF COMETS.
SECTION I.
SUPPOSED PHYSICAL INFLUENCES OF COMETS.
PAGB
The great comet of 1811 ; the comet wine — Prejudices and conjectures — Remark-
able comets and telescopic comets — Comets are continually traversing the
heavens . . . . . . . . . 495
SECTION II.
DO COMETS EXERCISE ANT INFLUENCE UPON THE SEASONS?
Study of the question by Arago — The calorific action of comets upon the earth
appears to be inappreciable — Comparison of the meteorological statistics of
various years in which comets did and did not appear — The meteorological
influence of a comet is not yet proved by any authentic fact . . 499
SECTION III.
PENETRATION OF COMETARY MATTER INTO THE TERRESTRIAL
ATMOSPHERE.
Is this penetration physically possible ? — Cometary influences, according to Dr.
Forster— Were the dry fogs of 1783, 1831, and 1834, due to the tails of
comets ? — Volcanic phenomena and burning turf beds ; their probable coin-
cidence with fogs — Probable hypothesis of Franklin — Dry fogs, atmospheric
dust, and bolides ........ 603
SECTION IV.
CHEMICAL INFLUENCES OF COMETS.
Introduction of poisonous vapours into the terrestrial atmosphere — The end of the
world and the imaginary comet of Edgar Poe ; Conversation of Eiros and Char-
mion — Poetry and Science ; impossibilities and contradictions . . 508
xxxm
CONTENTS.
CHAPTER XV.
SOME QUESTIONS ABOUT COMETS.
SECTION I.
ARE COMETS HABITABLE ?
The inhabitants of comets as depicted in the Plurality des Mondes of Fontenelle
—Ideas of Lambert respecting the habitability of comets— That comets are
the abode of human beings is a hypothesis incompatible with the received
facts of astronomy ....••••
SECTION II.
WHAT WOULD BECOME OF THE EARTH IF A COMET WEEE TO MAKE IT
ITS SATELLITE ?
Conditions of temperature to which the earth would be subjected if it were com-
pelled by a comet to describe the same orbit as the latter — The comets of
Halley, and of 1680, examined from this point of view — Extremes of heat
and cold: opinion of Arago: impossibility of living beings resisting such
523
SECTION III.
IS THE MOON AN ANCIENT COMET ?
Hypothesis of Maupertuis : the planetary satellites originally comets, which have
been retained by the attractions of the planets — The Arcadians and the moon
— Refutation of this hypothesis by Dionys du Se'jour . . . 628
TABLE I.
Elliptic Elements of the recognised periodical comets of the Solar system . 631
TABLE II.
General catalogue of the orbits of comets ..... 532
Note on the designation of comets, and on the catalogue of comets, by the Editor 545
xxxiv
LIST OF ILLUSTEATIONS.
PLATES.
The July 2 great comet of 1861, by De La Rue .
The great comet of 1861, by De La Rue, July 3
The great comet of 1843
Cheseaux's comet, 1744 ....
Donati's comet, 1858 . . .
Frontispiece
To face p. 248
„ . 152
„ . 212
220
WHOLE-PAGE WOODCUTS.
Coggia's comet, 1874
Forms of comets according to Pliny
Orbits of periodical comets
Donati's comet, September 1858 .
October 1858
To face p. 328
„ . 198
„ . 128
„ . 226
230
WOODCUTS IN TEXT.
FIG. PAGE
1 Phenomena of the Year 1000. Fac-simile of a drawing in the Theatrum
Cometicum of Lubienietzki . . . . . .21
2 Comet of 1528. Fac-simile of a drawing of Celestial Monsters from the
work of Ambrose Pare" . . . . . .' .22
3 Halley's comet on its apparition in 1066. From the Bayeux Tapestry . 24
4 Halley's comet in 684. Fac-simile of a drawing in the Chronicle of
Nuremberg ........ 25
5 Proper motion of a comet ; distinction between a comet and a nebula . 62
XXXV
LIST OF ILLUSTRATIONS.
PAGE
67
TIQ Maximum apparent movement of a comet and the earth .
7 Second Law of Kepler. The areas swept out by the radius vect ^
proportional to the time
8 Relation between the velocities and forms of orbits 74
9 Cometary orbits, elliptic, parabolic, and hyperbolic
10 Confusion of the arcs of orbits of different eccentricities in the neighbour-
hood of the perihelion .
11 12 13, 14 Determination of a cometary orbit: parabolic elements
16 Comparison of the eccentricities of the orbit of Faye's comet with that of
the planet Polyhymnia .
16 Halley's comet in 1835. 1. As seen by the naked eye October 24. 2.
As seen in the telescope the same day .
17 Encke's comet, at its passage in 1838, August 13
18 Brorsen's comet, as observed May 14, 1868, from a drawing of Bruhns . 120
19 Great comet of 1264, from Theatrum Cometicum of Lubienietzki . . 146
20 Great comet of 1811 . .... 151
21 Cometary nebulosities ; central condensation ; absence of tail and nucleus. 201
22 Encke's comet according to Mr. Carpenter . 202
23 Encke's comet, December 3, 1871, according to Mr. H. Cooper Key . 203
24 General direction of cometary tails . • . 207
25 Double tail of the comet of 1823 . . . 210
26 Double tail of the comet of 1850 . . ... 210
27 Comet of 1851 ........ 210
28 Sextuple tail of the comet of 1744, according to Che"seaux . . 211
29 Fan-shaped tail of the great comet of 1861, according to the observation
of June 30 and the drawing of Mr. G. Williams . . . 213
30 The two tails of the comet of 1861, according to Secchi, June 30 and
July 2 214
31 Winnecke's comet, June 19, 1868 . . . . . .216
32 Comet of P. Henry, August 26 and 29, 1873 . . . .217
33 The comet of 1264 218
34 Aspect of Donati's comet on December 3, 4, and 6, 1858, according to the
observations of M. Liais ...... 226
35 Variations of length in the principal tail of Donati's comet . . 227
36 Parabolic orbit of Donati's comet. Projection of the earth's orbit upon
the comet's orbit. Relative positions of the two bodies . . 229
37 Projection of the orbit of Donati's comet upon the plane of the ecliptic.
Relative positions of the earth and comet .... 229
38 Encke's comet, according to the observations of Schwabe. 1. October 19,
1838 ; 2. November 5 ; 3. November 10 ; 4. November 12 .242
39 Luminous sectors and aigrettes of Halley's comet, according to Schwabe.
(1) October 7, 1835 ; (2) October 11 ; (3) October 15 ; (4) October 21 ;
(5) October 22 ; (6) October 23 . . . . . .250
xxx vi
LIST OF ILLUSTRATIONS.
FIO. PAflE
40 Formation of luminous sectors and envelopes. Donati's comet, Sept. 8,
1858 . . . . . . . .251
41 Comet of I860, III. June 27, according to Bond. Aigrettes and en-
velopes . . . . . . . . ' 251
42 Luminous envelopes of Donati's comet. September 30, 1858 . . 252
43 The same comet. October 2. From a drawing by Bond . . 252
44 Formation of the luminous envelopes in Donati's comet. October 6 . 253
45 The same. October 8. Both from drawings by Bond . . . 253
46 Biela's comet after the duplication on February 21, 1846. According to
Struve . . . . . . . . 261
47 The twin comets of Biela at their return in 1852. According to Secchi . 263
48 The Olinda double comet on February 27, 1860, according to M. Liais . 270
49 The Olinda double comet on March 10, 1860, according to M. Liais . 271
50 The Olinda double comet on March 11, 1860, according to M. Liais . 272
51 Supposed phases of the comet of 1819, according to Oacciatore: observa-
tions of July 5 and 15 ....... 311
52 Comet of 1868, II. (Winnecke's.) From a drawing made by Mr.
Huggins . . . . . . ' . . .319
53 Spectra of the light of the comets of 1868, I. (Brorsen), and 1868, II.
(Winnecke) from the observations of Mr. Huggins: (1) Solar spectrum:
(2) spectrum of carbon spark taken in olive oil ; (3) spectrum of carbon
spark taken in olefiant gas; (4) spectrum of comet of 1868, II.; (5)
spectrum of Brorsen's comet, 1868, I.; (6) spectrum of an induction
spark ..... . . 320
54 Spectrum of the comet 1873, IV. (Henry's) (1) August 26; (2) Au-
gust 29 ......... 321
55 Coggia's comet, June 10, 1874, according to the drawing of M. G. Rayet 322
56 Spectra of the comet of 1874, III. (Coggia's), according to Father Secchi 331
57 Coggia's comet seen in the telescope on June 22, 1874, according to a
drawing by M. G. Rayet ...... 335
58 Coggia's comet on July 1, 1874, according to a drawing by M. G. Rayet 336
59 Coggia's comet on July 13, 1874, according to M. G. Rayet . . 337
60 Coggia's comet on July 14, according to M. G. Rayet . . . 338
61 Comet of 1618, according to Hevelius. Multiple nuclei . . . 341
62 Comet of 1661, according to Hevelius. Multiple nuclei . . . 341
63 M. Roche's theory of cometary phenomena. Limiting atmospheric sur-
face of equilibrium ....... 382
64 Flow of cometary matter beyond the free surface of the atmosphere. No
repulsive force ....... 383
65 Development of cometary tails, on the hypothesis of an intense repulsive
force. M. Roche's theory ...... 386
66 Development of cometary tails on the hypothesis of a feeble repulsive
force. M. Roche's theory . . . . . .386
67 Influence of a resisting medium upon the orbit of a comet . . 409
xxxvii
LIST OF ILLUSTRATIONS.
FIO. PAGE
68 Radial and tangential components of the repulsive force, according to M.
Faye . .... 412
69 Shooting stars of November 13-14, 1866. Convergence of the tracts, ac-
cording to A. S. Herschel and A. MacGregor .... 426
70 Orbits of the Meteoric Streams of November, August, and April, and of
the comets of 1866 and 1861 . . . . . .432
A Tracks of Meteors observed at the Royal Observatory, Greenwich, on the
night of November 13-14, 1866 . . . . .441
B Showing the rate of frequency of Meteors seen per minute at the Royal
Observatory, Greenwich, on the night of November 13-14, 1866 . 442
C Tracks of Meteors recorded by Professor Tachini at Palermo on August 8
to August 12, 1868 . . . . . . .443
D (1) Nucleus of the comet of 1618, observed telescopically by Cysatus.
(2) Comet of 1652, as seen December 27, according to Hevelius . 444
E Positions of Biela's comet relatively to the earth in 1798, 1838, and 1872 446
71 The orbit of the earth and that of Biela's comet in 1832. Relative posi-
tions of the two bodies ....... 462
72 Biela's comet at its node, October 29, 1832. Supposed position of the
comet at its least distance from the earth ... 463
73 Passage of the earth through the tail of the comet of 1861, on June 30 . 489
74 Positions occupied by the earth and the moon in the interior of the second
tail of the comet of 1861 ..... 489
75 Fan-shaped tail of the great comet of 1861 on June 30 . 491
X-xxvlii
CHAPTER I.
BELIEE3 AND SUPERSTITIONS
EELATIVE TO COMETS.
i;
SECTION I.
COMETS CONSIDERED AS PRESAGES.
Comets have been considered in all times and in all countries as signs, precursors of
fatal events — Antiquity and universality of this belief ; its probable origin —
Opinion of Seneca ; habitual and regular phenomena fail to attract the attention
of the multitude ; meteors and comets, on the contrary, make a profound im-
pression— The moderns in this respect resemble the ancients contemporary
with Seneca — The incorruptible heavens of the ancients, in contradistinction
to the sublunary or atmospheric regions ; stars and meteors — Inevitable confusion
of certain celestial or cosmical phenomena with atmospheric meteors.
IN all countries and in all times the apparition of a comet has
been considered as a presage: a presage fortunate or un-
fortunate according to the circumstances, the popular state of
mind, the prevailing degree of superstition, the imbecility of
princes or the calculation of courtiers. Science itself has
helped to confirm the formidable and terrible signification
most frequently accorded by common belief to the sudden and
unexpected arrival of one of these remarkable stars. Not two
centuries ago, as we shall shortly see, learned men and as-
tronomers of undoubted merit continued to believe in the
influence of comets over human events. What wonder, then,
if we should find existing in our own time, in the midst of
the nineteenth century, numerous vestiges of a superstition as
old as the world?
How has this superstition originated ? This is a question
we shall not undertake to resolve : we leave it to others more
3 B 2
THE WORLD OF COMETS.
learned and competent than ourselves in similar matters to
reply. Let us confine ourselves to a simple and by no means
new remark. The things which we see every day, the
phenomena which are constantly or regularly reproduced
under our eyes do not strike us, and fail to excite either
our attention or curiosity. D'Alembert has said : ' It is not
without reason that philosophers are astonished to see a stone
fall to the ground, and people who laugh at their astonishment
will upon the smallest reflection share it themselves.' Yes, it
is necessary to be a philosopher, or man of science, as we
should say at the present day; it is necessary to reflect in
order to diocover the why and the how of facts, of those at
least whose production is frequent and regular. The most
admirable phenomena remain unperceived. Habit blunts the
impression we derive from them and renders us indifferent.
As applied to comets, this idea has been perfectly expressed
by Seneca, at the commencement of Book vii. of his Qucestiones
Naturales: ' There is,' he observes, 'no mortal so apathetic, so
obtuse, so bowed down towards the earth, that he does not
erect himself and tend with all the powers of his mind towards
divine things, particularly when some new phenomenon makes
its appearance in the heavens. Whilst all above follows its
daily course, the recurrence of the spectacle robs it of its
grandeur. For man is thus constituted: that which he sees
every day, however admirable it may be, he passes with
indifference, whilst the least important things as soon as they
depart from the accustomed order captivate and interest him.
The whole choir of heavenly constellations under this immense
vault, whose beauty they diversify, fails to attract the attention
of the multitude ; but should anything extraordinary appear, all
faces are turned towards the heavens. The sun has spectators
only when he is eclipsed. The moon is observed only when
she undergoes a similar crisis. Cities then raise a cry of alarm,
4
COMETS CONSIDERED AS PRESAGES.
and everyone in panic fear trembles for himself. ... So much
is it in our nature to admire the new rather than the great.
The same thing takes place in respect to comets. If one of
these flaming bodies should appear of rare and unusual form,
everyone is anxious to see what it is ; all the rest are forgotten
whilst everyone enquires concerning the new arrival: no one
knows whether to admire or to tremble; for there are not
wanting people who draw from thence grave prognostics and
disseminate fears.'
Is it not with us to-day as with the contemporaries of
Seneca? Doubtless thoughtful and reflective minds yield
themselves readily to a sentiment of contemplative admiration
before the majestic spectacle of the heavens. The solemn
march of the heavenly bodies, the well-ordered harmony of
worlds, are for them the symbol of eternal laws governing the
universe; from the unalterability of these laws they derive
confidence. But 'the mass of the people ordinarily remains
indifferent before impassable and immutable nature. It is
reserved for one unusual apparition to rouse all from this
indifference, to awaken curiosity in some, fear in others, and,
if the phenomenon should be of unwonted proportions, ad-
miration in every one.
Moreover, whether it be a. comet, or any other remarkable
meteor, bolide, aurora borealis, or stone fallen from heaven,
the sentiments of fear inspired by these phenomena are
always the same, the superstitious interpretation similar, but
closely proportioned in degree to the brilliancy and the more
or less whimsical or extraordinary form of the apparition.
Amongst the Greeks and Romans, as we all know, a num-
ber of the most ordinary and familiar actions, singular ren-
contres, the cries of animals, the flight and the song of birds,
were looked upon as omens, as so many means made use of by
the gods to communicate with man, to warn him of their
5 <
THE WORLD OF COMETS.
decrees, to signify to him their thoughts and will. But they
regarded the importance of the warning as proportional to the
grandeur of the sign and the brilliancy of the phenomenon,
and it is not difficult to understand that comets amongst these
manifestations of the divine will appeared the most significant
and formidable.
A comet, moreover, not being a simple local phenomenon,
seen only by some, but exhibiting itself to all, brilliant as a
star, and of unusual dimensions, varying from day to day in
form, position and size, had all the appearance of a sign fraught
with significance to the entire people : this portent addressed
itself to those who played an important part in public affairs, and
concerned kings, or at least great personages. It had a certain
resemblance to the stars, which it sometimes surpassed in the
brilliancy of its light, but it differed from them in its erratic
course; it had a certain resemblance likewise to atmospheric
meteors, by its sudden appearance, and oftentimes as sudden
disappearance, and by the rapid changes to which it was
ordinarily subjected. The heavens, with their countless hosts,
the sun, the moon, the fixed stars and planets, were for the
ancients the domain of the incorruptible — cosli incorrupti.
Under this name it was the dwelling-place of the gods, the
habitation of the immortals. On the contrary, tjie air, the
atmosphere, the sublunary space — for the ancients it was all
one — was the region of meteors and of things corruptible and
fleeting; and in the same manner as the thunder-bolt of Jupiter
was the chosen instrument of his vengeance, comets were the
selected messengers of fate, sent to announce to mortals, on
the part of the gods, events that were inevitable.
In this confusion of certain celestial phenomena with
atmospheric meteors lies the chief source of the difficulty
experienced by the astronomers of antiquity and the Middle
Ages, and even of modern times, in solving the complicated
c
COMETS CONSIDERED AS PRESAGES.
problem of cometary movements. Down to the sixteenth
century, we shall see men of undoubted science refusing to
comets the quality of stars. They were confirmed in this
error alike by the prejudices we have just mentioned and by
the superstitious beliefs which are found to be so persistent
amongst all people and, as we have already said, in all times:
doubtless because these beliefs have the same foundation or
common origin, faith in the supernatural intervention of the
gods in human affairs.
But let us see what signification was given to the apparition
of comets by the ancient Greeks and Romans. It is a curious
and instructive side of cometary science; it will aid us further
on perhaps in comprehending the ideas entertained by their
astronomers concerning the physical nature of these stars.
SECTION II.
COMETS IN GREEK AND EOMAN ANTIQUITY.
The apparition of a comet or a bolide is a warning from the gods: the Iliad and the
^Eneid— Supposed physical influences of comets ; Earthquakes in Achaia ; sub-
mersion of Helice and Bura ; comet of the year 371— Comets, presages of happy
augury ; C?esar transported to the heavens under the form of a comet ; popular
credulity turned to account ; opinion of Bayle— Pliny, Virgil, Tacitus, Seneca—
The comet of the year 79 and the Emperor Vespasian— Comet of the year 400
and the siege of Constantinople.
A COMET is thought to have appeared in the last year of the
sieo-e of Troy. By Pingre" and Lalande it is considered an
apparition of the famous comet of 1680, and whilst the former
cites in support of his opinion a passage from Homer, the latter
draws attention to certain lines in the ^Eneid probably referring
to the same comet.* The following is the passage in the fourth
book of the Iliad to which Lalande refers : —
4 Thus having spoken, he urged on Athene already in-
clined; she hastening descended the heights of Olympus. As
the star which the son of wily Saturn sends, a sign either to
mariners or to a wide host of nations, and from which many
sparks are emitted: so Pallas Athene hastened to the earth
and leaped into the midst [of the army] ; and astonishment
seized the horse-breaking Trojans and the well-greaved Greeks
* In the time of Pingr^ and Lalande the period of revolution of the comet
of 1680 was believed to be 575 years. Encke has since assigned it the much
longer period of 8814 years.
8
COMETS IX GREEK AND ROMAN ANTIQUITY.
looking on. And thus said one to another : " Doubtless evil
war and dreadful battle din will take place again, or Zeus, the
arbiter of war amongst men, is establishing friendship between
both sides." ' *
In our opinion the star to which Homer compares Athene
was no comet, but a bolide, the explosion of which is frequently
attended with an emission of sparks, and can itself be seen
in broad daylight. This remark applies in all respects to
the verses of Virgil, who, moreover, mentions the noise of
the detonation, oftentimes similar, in explosions of bolides, to
the rumbling of thunder. Anchises, in the passage we now
quote, has just ceased to invoke Jupiter: —
' Scarcely had my aged sire thus said, when, with a sudden
peal, it thundered on the left, and a star, that fell from the
skies, drawing a fiery train, shot through the shade with a
profusion of light. We could see it, gliding over the high
tops of the palace, lose itself in the woods of Mount Ida, full
in our view, and marking out the way; then all along its
course an indented path shines, and all the place a great way
round smokes with sulphureous steam. And now my father,
overcome, raises himself to heaven, addresses the gods, and
pays adoration to the holy star.' f
The confusion above indicated is of frequent occurrence
amongst the ancient writers, with whom the Aurora Borealis,
bolides, and comets are evidently phenomena of the same
nature, and, so far as their supernatural interpretation is con-
cerned, this is not difficult to understand. Even in our day
the public is not more exact. Who can have forgotten the
impression caused by the splendid Aurora Borealis of Novem-
ber 24, 1870, during the Siege of Paris? That reddened glare
in the heavens, those shifting paths of light, were they not,
* Iliad, iv. 52-84. t ^Eneid, ii. 674-709.
THE WORLD OF COMETS.
for weak and credulous minds, under the stimulus besides of
passing events, certain presages of the blood about to be shed?
A comet would have produced a similar effect.
Be this as it may, the meteor, be it either bolide or
comet, is in the eyes of all beholders, both in Virgil and
Homer, a presage, or warning from Jupiter,-a sign, the pre-
cursor of events auspicious or inauspicious, according to the
interpretation or circumstances.
Three hundred and seventy-one years before our era a very
brilliant comet appeared, which Aristotle has described, and
which Diodorus Siculus refers to in the following terms : * In
the first year of the hundred and second Olympiad, Alcisthenes
being then Archon of Athens, several prodigies announced
to the Lacedaemonians their approaching humiliation : a burn-
ino- torch of extraordinary size, which was spoken of as a
fiery beam, appeared for several nights.' This comet, of which
we shall speak again, had more than one claim to notice.
According to Ephorus it divided into two ; and about the time
of its apparition the earthquakes took place which caused
Helice and Bura, two towns of Achaia, to be swallowed up by
the sea. Comets, therefore, for the ancients were not only
the precursors of fatal events, but they had direct power to
occasion them. Such is certainly the opinion of Seneca when
he remarks : ' This comet, so anxiously observed by everyone,
because of the great catastrophe which it produced as soon as it
appeared, the submersion of Bura and Helice.'
Comets not only announced fatal events, disasters, and
wars; portents of evil for some, they were naturally presages of
happy augury for others. Thus, according to Diodorus Siculus
and Plutarch, the comet of the year B.C. 344 was for Timoleon
of Corinth a token of the success of the expedition which he
directed that year against Sicily. ' The gods by an extra-
ordinary prodigy announced his success and future greatness:
10
COMETS IN GREEK AND ROMAN ANTIQUITY
a burning torch appeared in the heavens throughout the night
and preceded the fleet of Timoleon until it arrived off the
coast of Sicily.'
The births and deaths of princes, especially of those remem-
bered in history for the evil they have done, could not fail to be
distinguished by the apparition of prodigies, and by comets,
the most striking of all prodigies. Thus the comets of B.C. 134
or 137 and B.C. 118 were referred, the former to the birth and
the latter to the accession of Mithridates, and the comet of
the year B.C. 43 was supposed to be nothing less than the soul
of CaBsar transported to the heavens. Bodin ( Universce Naturae
Tkeatrum) attributes to Democritus the opinion that such is,
in fact, the part performed by some of these stars, and con-
fesses that he is not far from sharing the same opinion. ' I
reflect,' he says, ' upon the idea of Democritus, and I am led to
believe with him that comets are the souls of illustrious per-
sons, which, after having lived upon the earth for a long suc-
cession of ages, ready at last to perish, are borne along in a sort
of triumph, or are called to the starr-y heavens, as brilliant
lights. This is why famine, epidemic maladies, and civil wars
follow the apparition of comets; cities and nations then find
themselves deprived of the help of those excellent leaders who
strove to allay their intestine troubles.' We willingly place
amongst the beliefs arid superstitions of the ancients this
triumphant explanation of the supposed disasters which, ac-
cording to the universal opinion, were certain to follow the
apparition of a comet. Nor is it in all probability of earlier
date than the sixteenth century, for it is very likely that
Bodin calumniated Democritus and was himself its true
author. Pingre", remarking on the above passage and the
apotheosis of Ca3sar, observes with justice: 'The preceding
should be added to the number of base and indecent flatteries,
rather than be classed among philosophical opinions.'
11
THE WORLD OF COMETS.
Less than a century after Bodin we find Bayle protesting
against this superstition, which appeared singular indeed to a
man so enlightened as he who has been compared to Montes-
quieu. In his Pensees sur la Comete, in which so much good
sense is blended with so much irony, Bayle ingeniously shows
with what skill popular credulity was turned to account, and
how the same comet was made to subserve several ends.
' Augustus,' he says, ' from policy, was well pleased that the
people should believe it to be the soul of Caesar ; because it
was a great advantage for his party to have it believed that
they were pursuing the murderers of a man who was then
amongst the gods. For this reason he caused a temple to be
built to this comet, and publicly declared that he looked upon
it as a very auspicious omen. . . . Those who were still repub-
lican at heart said, on the contrary, that the gods testified by
it their displeasure that the liberators of their country were
not supported.'
In the Natural History of Pliny we find several passages
attesting the terrible significance attached to comets by the
ancients. ' A comet,' he observes, ' is ordinarily a very fearful
star; it announces no small effusion of blood. We have seen
an example of this during the civil commotion in the consulate
of Octavius.' This refers to the comet which appeared B.C. 86.
The following alludes to the comet of B.C. 48, and perhaps
no less to the apparition of remarkable bolides and Aurorse
Boreales : ' We have, in the war between Caesar and Pompey,
an example of the terrible effects which follow the apparition
of a comet. Towards the commencement of this war the
darkest nights were made light, according to Lucan, by un-
known stars; the heavens appeared on fire, burning torches
traversed in all directions the depths of space; the comet, that
fearful star, which overthrows the powers of the earth, showed
its terrible locks.'
12
COMETS IN GREEK AND ROMAN ANTIQUITY.
Virgil, at the eud of the first Georgic, expresses in his
harmonious language all the horror caused in the super-
stitious minds of the vulgar by the prodigies so skilfully turned
to account by politicians arid sceptics. After speaking of the
prognostics which may be drawn from the aspect of the setting
sun in reference to the weather, he adds : —
4 Who dares to call the sun deceiver? He even forewarns
often that hidden tumults are at hand, and that treachery and
secret wars are swelling to a head. He also pitied Rome at
Caesar's death, when he covered his bright head with murky
iron hue, and the impious age feared eternal night; though
at that time the earth too, and ocean's plains, ill-omened dogs,
and presaging birds, gave ominous signs. How often have we
seen ^Etna from its burst furnaces boil over in waves on the
lands of the Cyclops and shoot up globes of flame and molten
rocks ! Germany heard a clashing of arms over all the sky ;
the Alps trembled with unwonted earthquakes. A mighty
voice, too, was commonly heard through all the silent groves,
and spectres strangely pale were seen under the cloud of night ;
and the very cattle (Oh horrible!) spoke; rivers stopped their
courses, the earth yawned wide ; the mourning ivory weeps in
the temples, and the brazen statues sweat. Eridanus, king of
rivers, overflowed, whirling in mad eddy whole woods along,
and bore away the herds with their stalls over all the plains.
Nor at the same time did either the fibres fail to appear
threatening in the baleful entrails, or blood to flow from the
wells, and cities to resound aloud with wolves howling by
night. Never did more lightnings fall from a serene sky nor
direful comets so often blaze.' *
All these prodigies, this mixture of facts natural and true,
and the whimsical beliefs of popular credulity, are for the poet
* Georgic, i. 463-488.
13
THE WORLD OF COMETS.
testimonies of the anger and vengeance of the gods, fore-
runners of fresh disasters, the precursors of that battle of
Philippi in which Roman armies inflamed by civil discord are
about to encounter and shed each other's blood. Nature acts
in unison with man, and her manifestations are a reflex of his
fury. Everything, moreover, concurs to render the divine
intervention striking ; earthquakes, volcanic eruptions, and
inundations. The comets and bolides with which Virgil con-
cludes his enumeration appear to be the supreme signs of this
menacing intervention:
Non alias coelo ceciderunt plura sereno
Fulgura ; nee diri toties arsere cometae.
Later on, comets were not only presages : they became
pretexts for the persecutions of imperial tyranny. Thus,
Tacitus says, in regard to the comet of the year 64 : 'At
the close of this year people discoursed only of prodigies,
the forerunners of approaching calamities ; of thunderbolts
more frequent than at any other epoch, and of the apparition
of a comet, a kind of presage that Nero always expiated with
illustrious blood.' Several comets, in fact, appeared during
the reign of this monster, and it is concerning one of them
that Seneca had the audacity to say, ' that having appeared in
the reign of Nero, it has removed infamy from cornets.' It
does not seem, however — and we shall find other proofs of it
later on — that the author of the Qacestiones Naturales shared
the prevailing prejudices on the subject of comets. He does
not deny that they cause disasters, but he manifestly inclines
towards a physical explanation of these phenomena. Speaking
of the comet of the year 62, he observes : ' The comet which
appeared under the consulate of Paterculus and Vopiscus has
been attended with the consequences that Aristotle and Theo-
phrastus have attributed to this kind of star. Everywhere
14
COMETS IN GREEK AND ROMAN ANTIQUITY.
there have been violent and continual storms: in Achaia and
Macedonia several towns have been overthrown by earth-
quakes.'
Let us conclude what more we have to say of the super-
stitious beliefs of the ancients concerning comets with the
mention of two or three famous apparitions; they will suffice
to show that from the most remote antiquity down to the
Middle Ages, from the erroneous ideas of the pagans to those
of Christian nations, during this long dark night of history we
pass without interruption or sensible modification.
In the year 69, according to Josephus, several prodigies
announced the destruction of Jerusalem. 'Amongst other
warnings, a comet, one of the kind called Xiphias, because
their tails appear to represent the blade of a sword, was seen
above the city for the space of a whole year.'
Pingre* quotes, in reference to the comet of the year 79, this
curious passage from Dion Cassius, which proves that there were
esprits forts even amongst the Roman emperors : ' Several
prodigies preceded the death of Vespasian; a comet was for a
long time visible ; the tomb of Augustus opened of itself. When
the physicians reproved the Emperor Vespasian for continuing
to live as usual and attend to the business of the state, al-
though attacked by a serious malady, he replied, " It is fitting
that an emperor should die standing." Perceiving some cour-
tiers conversing together in a low tone of voice about the
comet, " This hairy star," he remarked, " does not concern me;
it menaces rather the King of the Parthians, for he is hairy
and I am bald." Feeling his end approach, UI think," said he,
" that I am becoming a god."
The death of the Emperor Constantine was announced by
the comet of the year 336.
In the year 400 the misfortunes with which Gainas menaced
Constantinople were so great, say the historians Socrates and
15
THE WORLD OF COMETS.
Sozomenes, that they were announced by the most terrible
comet mentioned in history; it shone above the city, and
reached from the highest heavens to the earth. The same
comet was also regarded as the presage of a plague which broke
out about the same time.
Lastly, the invasions of barbarians, at a time when moral
disorder and anarchy of ideas were in unison with the disor-
ganisation of the Empire, could not fail to be signalised by
various prodigies, birds of evil augury, frequent thunderbolts,
monstrous hailstones, fires, and likewise apparitions of comets,
* that spectacle which the earth has never seen with impunity.'
In the Middle Ages, therefore, we shall find that beliefs in the
supernatural and the intervention of the gods in human affairs
are further strengthened and increased by the mysticism which
the ascendency of religious ideas tended to foster in the minds
of the people.
16
SECTION III.
THE COMETS OF THE MIDDLE AGES.
Prevalence of popular superstitions — Comets announce wars, plagues, the deaths of
sovereigns — Terrors of the year 1000; comets and the end of the world — Gian
Galeazzo Visconti and the comet of 1402 — Ambrose Pare" ; celestial monsters —
Halley's comet and the Turks; origin of the Anyelus de Midi — The comet of
1066 and the conquest of England by the Normans ; apostrophe to the comet by a
monk of Malmesbury.
IF a complete history were desired of all the superstitions
which, during the Middle Ages and in modern times, have
obtained with respect to comets, it would be necessary to
pass in review every apparition of these stars, together with
such incidental phenomena as the Aurora Borealis, new and
temporary stars, bolides, &c., all of which have been con-
verted by popular credulity into as many prodigies. In-
teresting in a scientific point of view, this long enumeration
derived from the naive chronicles of the time, the only docu-
ments available in the absence of a more complete and intelli-
gent record, would be but a tedious study of human errors ; a
constant and monotonous repetition of the same absurd beliefs.
To this state of things savants have themselves contributed,
as at the epoch when these voluminous records were compiled
cometary influences were still believed in, and the erudite of
the day shared the universal prejudice.
I will here limit myself to a few characteristic traits of this
17 C
THE WORLD OF COMETS.
tenacious superstition, in order to exhibit the progress, I was
about to say the revolution, of ideas which has taken place
under the increasing influence of science, and more especially
of astronomy and physics. Wherever the light of science has
been able to make its way the phantoms of the supernatural
have vanished, and the most extraordinary apparitions, even if
they continue unexplained, are no longer regarded as prodigies,
presages, or Divine manifestations, but natural phenomena con-
cerning which all men of science, without exception, are at one
in their endeavour to trace the laws that govern them.
Let us come now to the facts we have to mention.
In ancient times, especially amongst the Greeks, comets, as
it has been seen, were not invariably regarded as of evil omen.
The darker and more gloomy spirit of the Middle Ages only
saw in these apparitions the announcement of terrible events,
wars, pestilence, and especially the deaths of sovereigns. The
comet of 451 or 453 announced the death of Attila, and the
comet of 455 that of the Emperor Yalentinian; comets ap-
peared successively to announce the death of Meroveus in 577,
of Chilperic in 584, of the Emperor Maurice in 602, of Mahomet
in 632, of Louis le De'bonnaire in 837, of the Emperor Louis II.
in 875. That the apparition of comets was connected with the
death of the great is an idea so widely spread that many chroni-
clers appear to have recorded, perhaps in good faith, comets
which were never seen ; such, according to Pingre, was the
comet of 814, which presaged the death of Charlemagne.
In the year 1024 a comet appeared, an augury, it was sup-
posed, of the death of the King of Poland, Boleslas I. ; an
eclipse of the sun and a comet marked in 1033 that of Robert,
King of France ; comets appeared in 1058, the year of the death
of Casimir, King of Poland; in 1060, the year in which died
Henry I., King of France, and in the years 1181, 1198, 1223,
1250, 1254, 1264, 1337, 1402, 1476, 1505, 1516, and 1560.
18
THE COMETS OF THE MIDDLE AGES.
Under these respective dates we find the deaths of the follow-
ing sovereigns : Pope Alexander III. ; Richard I., King of
England; Philip Augustus, King of France; the Emperor
Frederick, deposed and excommunicated; Pope Urban IV.;
Gian Galeazzo Visconti, Duke of Milan; Charles the Bold;
Philip I. of Spain; Ferdinand the Catholic; and Francis II.,
King of France. This list might be considerably extended.
Amongst the chroniclers or historians who relate these coin-
cidences we find no shadow of a doubt as to the certainty
or signification of the presage. The mention of these signs,
forerunners of the deaths of sovereigns, very frequently occurs
with a curious naivete", of which we will give two or three
examples.
1 At the commencement of July,' says an old French chro-
nicle, ' a little before the half, appeared a sign in the heavens
called a comet denoting a convulsion of the kingdom ; for
Philip, the king, who for a long time had lain ill of a quartan
ague, at Mantua, closed his last day on the 14th of July, 1223.'
Gian Galeazzo Visconti was sick when the comet of 1402
appeared. As soon as he perceived the fatal star he despaired
of life : ' For,' said he, ' our father revealed to us on his death-
bed that, according to the testimony of the astrologers, a similar
star would appear for eight days at the time of our death.'
' This prince was not deceived,' adds the historian, from whom
we borrow this account; 'surprised by an unexpected malady,
he died a few days after.' Another chronicler gives us to un-
derstand that the comet only appeared when Galeas was already
attacked by the malady of which he died. But the faith of the
duke in the celestial warning was not less complete. ' At this
time a great comet was seen. Galeas was told of it. His
friends helped him to leave his bed; he saw the comet, and
exclaimed, " I render thanks to God for having decreed that
my death should be announced to men by this celestial sign."
19 o 2
THE WORLD OF COMETS.
His malady increasing, he died shortly afterwards, at Marig-
nan, on the 3rd of September.'
Pino-re", in quoting the first of these accounts, observes that
the unexpected malady of Galeas might well have been occasioned
by the chimerical fear of this prince; he might have added,
or aggravated. This simple remark of the Canon of St. Ge-
nevieve sufficiently marks the difference of the times. Till
the eighteenth century the writers who record the coincidences
of comets and. great events have implicit faith in them, and
naively describe as a self-evident fact the connexion between
the comet and the event itself. Pingre, writing in the eighteenth
century, less than a century after the labours of Newton, and
in quest of dates to enable him to calculate various cometary
orbits, esteems it fortunate that in these times of ignorance
such absurd beliefs should have existed, as without them
history perhaps would never have recorded one of these ap-
paritions so valuable to science.
There have been degrees nevertheless, according to the
times, in the superstitious terror created by the apparition of a
comet; this terror was also proportioned to the degree of bril-
liancy of the star, the magnitude of its tail, and the more or
less singular form of the coma and luminous appendage. In
the year 1000, at that melancholy epoch when the end of the
world was so confidently looked for, the most simple pheno-
mena, if unexpected, must have assumed terrible proportions.
About this time we are told of earthquakes, and a comet was
visible for the space of nine days. ' The heavens having
opened, a kind of burning torch fell upon the earth, leaving
behind it a long train of light similar to a flash of lightning.
Such was its light that it frightened not only those who were
in the open country but those who were within doors. As
this opening in the heavens closed imperceptibly there be-
came visible the figure of a dragon, whose feet were blue, and
20
THE COMETS OF THE MIDDLE AGES.
whose head seemed continually to increase.' This evidently
relates to the apparition of a bolide, and also perhaps to an
Aurora Borealis, but not to the comet, whose apparition lasted
nine days.
The drawing of these ' frightful ' meteors, which we here
reproduce from the Tlieatrum Cometicum of Lubienietzki, is
interesting in various respects. It shows the height to which
imagination can attain under the stimulus of terror; it proves
also the little value to be attached, scientifically speaking, to
the descriptions of the time, whether written or portrayed.
This drawing is comparatively modern, probably of much later
date than the epoch at which the apparition represented by it
took place; but the next which we give (fig. 2) is taken from
Fig. 1. — Phenomena of the Year 1000. Fac-simile of a drawing in the Tfitatrum
Cometicum of Lubienietzki.
a work by Ambrose Pare, a contemporary of the apparition.
The decapitated heads, the sabres, the arms which accompany
the drawing of the hairy star, are only the translation of
what the over-excited popular imagination believed itself to
have seen in comets or other meteors, signs from heaven.*
* In his admirable work The Universe our late learned naturalist M. F. A.
Pouchet with much justice remarks, in a note : ' In Ambrose Pare may
be seen to what extent the mightiest minds of these latter centuries allowed
themselves to be led astray on the subject of comets. The illustrious surgeon,
who was by no means superstitious, gives in his important work the most fan-
21 /
THE WORLD OF COMETS.
Observe in what terms the historian Nicetas describes the
comet (or meteor) of the year 1182: ' After the Romans were
driven from Constantinople a prognostic was seen of the ex-
cesses and crimes to which Andronicus was to abandon him-
self. A comet appeared in the heavens similar to a writhing
Fig. 2.— Comet of 1528. Fac-simile of a drawing of Celestial Monsters from the work of
Ambrose Pare.
tastic drawings of some of these bodies. In his chapter entitled Celestial Monsters
Ambrose Pare speaks of comets as hairy, bearded, buckler- shaped, lance-shaped,
dragon-like, or resembling a battle of the clouds. And he in particular describes
and represents in all its details a blood-red comet which appeared in 1528 (the
figure above represented (fig. 2). ' This comet,' said he, ' was so horrible, so
frightful, and it produced such great terror in the vulgar, that some died of fear,
22
THE COMETS OF THE MIDDLE AGES.
serpent ; sometimes it extended itself, sometimes it drew itself
in; sometimes, to the great terror of the spectators, it opened
a huge mouth; it seemed that as if, thirsting for human blood,
it was upon the point of satiating itself.'
* Comiers,' says Pingre", ' makes a horrible comet appear in
the month of October, 1508, very red, representing human
heads, dissevered members, instruments of war, and in the
midst a sword.' May it not be, with an error of date on the
part of one or other of the chroniclers, the comet of which we
have spoken, and a fac-simile of which we have reproduced in
fig. 2?
Under the heading of periodical comets we shall see that
one of the most famous in history is that which is now called
Halley's comet, from the name of the astronomer who calcu-
lated and first predicted its return. This comet has, in fact,
made its appearance twenty-four times within sight of the
earth since the year 12 before our era. the most remote date on
record of its apparition. Let us here transcribe, according to
Babinet, the most remarkable particulars of the events which
have been connected with it by popular belief.
' The Mussulmans, with Mahomet at their head, were be-
sieging Belgrade, which was defended by Huniades, surnamed
the Exterminator -of the Turks. The comet of Halley appeared,
and the two armies were alike seized with fear. Pope Calix-
tus III., himself struck with the general terror, ordered public
prayers to be offered up, and launched a timid anathema against
the comet and the enemies of Christianity. He instituted the
prayer called the Angelas de Midi, the use of which still con-
and others fell sick. It appeared to be of excessive length, and was of the colour
of blood. At the summit of it was seen the figure of a bent arm, holding in its
hand a great sword, as if about to strike. At the end of the point there were
three stars. On both sides of the rays of this comet were seen a great number
of axes, knives, blood-coloured swords, among which were a great number of
hideous human faces, with beards and bristling hair.'
23
THE WORLD OF COMETS.
tinues in all Catholic churches. The Franciscans brought
40,000 defenders to Belgrade, besieged by the conqueror of
Constantinople, the destroyer of the Empire of the East. The
battle took place, and lasted two days without intermission.
This conflict of two days caused the loss of more than 40,000
combatants. The Franciscans, without arms, crucifix in hand,
appeared in the foremost ranks of the defenders, invoking the
exorcism of the Pope against the comet, and turned against
the enemy the Divine anger of which no man at this time
doubted. What primitive astronomers ! '
Fig. 3.— Halley's Comet on its apparition in 1066. From the Bayeux Tapestry.
But let us go back to an earlier date in the history of this
comet. It appeared in the month of April 1066. ' The Normans
had at their head their Duke William, since surnamed the Con-
queror, and were ready to invade England, the throne of which
was at that time usurped by Harold in spite of the faith sworn
to William.' That the comet was the precursor of the Conquest
no one doubted. A new star, a new sovereign. Nova stetta,
novus rex ! Such was the proverb of the time. The chroni-
clers say unanimously, ' The Normans, guided by a comet,
24
THE COMETS OF THE MIDDLE AGES.
invaded England.' Fig. 3 reproduces from the celebrated
Bayeux tapestry, attributed to Queen Matilda, wife of Wil-
liam the Conqueror, the episode in which the apparition of
the comet appears.
Halley's comet, by its apparition in 1066, gave rise to the
objurgations of the monk of Malmesbury, which have been
quoted by Pingre* from an old English chronicle : ' Seeing his
country on the point of being attacked on the one side by Harold,
King of Norway, on the other side by William, and judging
that bloodshed would
ensue, "Here art thou,
then," said he, apo-
strophising the comet,
" here art thou, source
of the tears of many
mothers. Long have
I seen thee; but now
thou appearest to me
more terrible, for thou
menacest my country
with complete ruin."
Going back further
still, we find that Hal-
ley's comet is that which announced the death of Louis le
Delxmnaire, which came to pass three years later. Lastly, the
comet of 684 (fig. 4) is also one of its apparitions.
We will say nothing of the famous comet of 1556, to the
influence of which was long attributed the abdication of the
Emperor Charles V., because it happens that the celebrated
emperor had already descended from the throne when the
comet made its appearance. We shall have* occasion to speak
further on of the announcement of its return between 1848 and
1860, and of its non-reappearance.
25
Fig. 4. — Halley's Comet in 684. Fac-simile of a
drawing in the Chronicle of Nuremberg.
SECTION IV.
COMETS FROM THE RENAISSANCE TO THE PRESENT DAY.
Slow improvement in the beliefs relative to comets— Bayle's remarks upon the comet
of 1680 — Passage from Madame de Sevigne"s letter referring to this comet and
the last hours of Mazarin— In the eighteenth century belief in the supernatural
exchanged for belief in the physical influence of comets — Remains of cometary
superstitions in the nineteenth century — The comet of 1812 and the Russian
campaign ; Napoleon I. and the comet of 1769 ; the great comet of 1861 in Italy.
WE have just seen that the superstitious ideas of the Middle
Ages were yet dominant in the height of the Renaissance,
since a man of learning like Ambrose Pare — no astronomer, it
is true — could attribute to comets the same malign influences
as those ascribed to them in the year 1000, when the end of
the world was confidently expected.* Nor could it be other-
wise, science not having then assigned to comets, in common
with other extraordinary meteors, their true place in the order
of nature.
Little by little, however, healthier ideas make their way,
and to the supernatural influence of comets we shall now see
gradually succeed in the minds of men of science and the
[* Milton has finely expressed the popular superstition with regard to comets
in the well-known lines —
' On the other side,
Inwnsed with indignation, Satan stood
Unterrified ; and like a comet burned,
That fires the length of Ophiuchus huge
In the arctic sky, and from his horrid hair
Shakes pestilence and wax.'— Paradise Lost, book ii. — ED.]
26
COMETS FROM THE RENAISSANCE TO THE PRESENT DAY.
more enlightened of the people the idea of an influence purely
physical, at first under the form of simple hypotheses, and
afterwards as a probability deduced from observations and facts.
This progress was slow, like that of cometary astronomy, and
owed much of its advance to the assistance of men of original
thought, who, without being astronomers, were yet conversant
with the scientific knowledge of their time.* Such was Bayle.
We have already quoted several passages from his Pensees sur
la Comete, and we will now complete what still remains to be
said in reference to this essay. ^
The Pensees diverses ecrites a un professeur de Sorbonne
were composed during the public excitement caused in France
and Europe by the apparition of the famous comet of December
1680. From the beginning Bayle adopts the opinion of Seneca,
and thus renews the train of rational and sound ideas. ' Comets,'
he remarks, ' are bodies subject to the ordinary laws of nature,
and not prodigies amenable to no law.' Supposing his corre-
spondent to share the current prejudices of the time, he is
astonished that so great a doctor should nevertheless suffer
himself to be carried along with the stream, and imagine like
the rest of the world, in spite of the arguments of the chosen
few, that comets are heralds-at-arms sent by God to declare
war against the human race,
He then examines the value of the historical testimonies
which different writers have applied to the support of the
current prejudice on comets.
1 The testimony of historians,' he remarks, ' proves only
* The following anecdote which we borrow from Bayle proves that the wits
of the seventeenth century began to treat with ridicule this long-cherished
superstition. ' It seems to me,' says M. de Bassompierre, writing to M. de Luynes,
in 1621, shortly after the death of Philip III., 'that the comet we laughed at
at St. Germain is no laughing matter, as it has buried in two months a pope, a
grand duke, and a king of Spain.' A belief which is expressed in these terms
may be considered as drawing to its end.
27 '
THE WORLD OF COMETS.
that comets have appeared, and that afterwards there have
been many disorders in the world, which is very far from
proving that the former are to be looked upon as the cause
or the prognostic of the latter, unless we are willing to admit
that a woman who never looks out of window in the Rue St.
Honore" without seeing carriages pass along the street is to
imagine that she is the cause of their passing, or that when she
shows herself at the window it is a sign to the whole quarter
that carriages will soon pass.'
Bayle next attacks astrology and its pretended princi-
ples, as the source of all the extravagant beliefs relative to
heavenly phenomena; and, indeed, prejudices in respect to
comets form but a portion of the whole, and are contained in
a separate chapter, which might well be entitled Cometary
Astrology.
' The details of cometary warnings, resting only upon the
principles of astrology, cannot fail to be ridiculous, because
there never has been anything more impertinent, more chi-
merical or more ignominious to human nature, to the eternal
shame of which it must be related, that there have been men
base enough to deceive others under the pretext of knowing
the affairs of heaven, and men foolish enough to believe in
them even to the extent of instituting the office of Astrologer,
and of not daring to wear a new coat, or plant a tree, without
the approbation of that functionary.
' The astrologer will tell you to what people, or to what
animals, the cornet has reference, and the kind of evil that may
be expected. In Aries it signifies great wars and mortality,
the fall of the great and the exaltation of the little, together
with fearful droughts in places under the dominion of that
sign. In Virgo it signifies dangerous childbirth, imprison-
ments, sterility and death amongst women. In Scorpio, in
addition to the preceding evils, reptiles and innumerable locusts.
28
COMETS FROM THE RENAISSANCE TO THE PRESENT DAY.
In Pisces disputes concerning points of faith, frightful appari-
tions in the air, wars and pestilence among the great, etc. . . .
* It is not in our own time only that astrologers have
reasoned upon such extravagances. The same thing pre-
vailed in the time of Pliny. "It is," says he, "thought to be
a matter not unimportant whether comets dart their beams
towards certain quarters, or derive their power from certain
stars, or represent certain things, or shine in particular parts
of the heavens. If they resemble a flute, the omen relates to
music; when they appear in certain parts of a sign, the omen
has reference to the immodest; if they are so situated as to form
an equilateral triangle or a square with some of the fixed stars,
they are addressed to learning and wit. They distribute poison
when they appear in the head of either the Northern or the
Southern Serpent.' (Pliny, book ii. chap, xxv.)
Bayle cites a remark attributed to Henry IV. which might
be applied, at the present day, to many so-called predictions.
Speaking of the astrologers who had forewarned him of his
death, Henry IV. is said to have exclaimed, ' They will be
right some day, and the public will remember the one pre-
diction that has come true, better than all the rest that have
proved false.'
The letter of the celebrated writer is long; it touches
upon very many considerations which, though of interest as
regards the history of ideas at the end of the sixteenth century,
would appear in the present day far removed from our subject;
but the philosophic thought which has inspired him is always
true. It may be summed up in these eloquent lines, the last
that we shall quote : —
' The more we study man the more does it appear that
pride is his ruling passion, and that he affects grandeur even
in his saddest misery. Mean and perishable creature that
he is, he has been able to persuade himself that he cannot die
29
THE WORLD OF COMETS.
without disturbing the whole of nature and obliging the heavens
to put themselves to fresh expense to light his funeral pomp.
Foolish and ridiculous vanity ! If we had a just idea of the
universe we should soon comprehend that the death or birth
of a prince is so insignificant a matter, compared to the whole
of nature, that it is not an event to stir the heavens.'
Madame de SevigmS, writing on January 2, 1681, to the
Comte de Bussy, mentions the same comet, then in sight, and
concludes with a remark which in reality is the same as Bayle's.
The following is the passage : —
'We have here a comet — it has the most beautiful tail that
could possibly be seen. All the greatest personages are alarmed,
and firmly believe that heaven, occupied with their loss, is
gi vino- intelligence of it by this comet. It is said that Cardinal
O O O V
Mazarin being despaired of by his physicians, his courtiers con-
sidered it necessary to honour his last hours by a prodigy, and
to tell him that a great comet had appeared which filled them
with alarm for him. He had strength enough to laugh at them,
and jestingly replied that the comet did him too much honour.
In truth, everyone should say the same, and human pride does
itself too much honour in believing that when perforce we die
it is a great event amongst the stars.'
At the present day what man of education, what enlight-
ened mind would fail to subscribe to the views of the cele-
brated author and the spirituelle marquise? Nevertheless
false beliefs relative to comets, celestial and even atmospheric
meteors, are not entirely destroyed. We might have found
traces of them in the last century, but in an epoch so
favourable to science, we must seek under another form for
the errors of which we have given a rapid sketch from the
most ancient down to comparatively modern times ; and in
the chapter which we shall devote to the possible influences
of comets upon the earth it will be seen that if the popular
30
COMETS FROM THE RENAISSANCE TO THE PRESENT DAY.
fears were then of a different kind they were none the less
-vivid. In our nineteenth century these fears have been openly
revived; the idea that the end of the world could be brought
about by the meeting of the earth and a cornet has found
minds disposed to receive it with blind acceptance. Further,
the old superstition of the supernatural influence or signi-
fication of comets is always rife amongst the ignorant masses
of the people, whose minds remain unaffected by the advance
of science, because to them science is a dead letter. The
following is a fact which occurred in Russia, and hardly more
than sixty years ago: —
' It was not by the exchange of diplomatic notes that the
inhabitants of Moscow derived a presentiment of some ap-
proaching calamity. The famous comet of 1812 first gave
them warning of it. Let us see what reflections it inspired in
the minds of the Abbess of the Dievitchi Monastir, and the nun
Antonina, formerly the slave of the Apraxines. ' One evening,
as we were on our way to a commemorative service at the
Church of the Decollation de Saint-Jean, I suddenly perceived
on the other side of the church what appeared to be a resplen-
dent sheaf of flame. I uttered a cry and nearly let fall the
lantern. The Lady Abbess came to me and said, " What art
thou doing? What ails thee?" Then she stepped three
paces forward, perceived the meteor likewise, and paused a
long time to contemplate it. " Matouchka" I asked, "what
star is that? " She replied, ult is not a star, it is a comet."
I then asked again, "But what is a comet? I have never
heard that word." The mother then said, " They are signs in
the heavens which God sends before misfortunes." Every
night the comet blazed in the heavens, and we all asked our-
selves, what misfortunes does it bring ? ' — lLa Grande Armee
a Moscou dapres les temoignages moscovites? — Revue des Deux
Mondes, July 1, 1873.
31 i
THE WORLD OF COMETS.
Can anyone deny that such credulity exists at the present
day and elsewhere than in Russia? Are there not persons
still who believe that the great comet of 1769, which appeared
in the year that Napoleon was born, presaged the era of war
which drenched in blood the end of the eighteenth century
and the beginning of the nineteenth, and all the disasters
which that too famous despot let loose on Europe and at last
upon France herself? Have we not seen quite recently, in 1861,
when the great comet of that year appeared, how it was cur-
rently reported in Italy, and doubtless elsewhere, that the
new star was a sign of the speedy return of Francis II. and
his restoration to the throne of the Two Sicilies ; and also that
it presaged the fall of the temporal power and the death of
Pope Pius IX.?
We ought not to be astonished at the persistence of these
superstitions, which only the spread of science can annihilate
for ever. After seeing, in the following chapter, with what
great difficulty true ideas on the subject of comets, suspected
centuries ago, have achieved their final victory, we shall not be
surprised to find that errors still remain in our own nineteenth
century, in the midst of what we regard as enlightened popu-
lations, but which will never be truly enlightened until
primary instruction shall have given to them more definite
notions of physics, natural history, and astronomy.
32
CHAPTEE II.
COMETAEY ASTRONOMY UP TO THE TIME OF NEWTON,
33
, D
SECTION I.
COMETS AND THE ASTRONOMERS OF EGYPT AND CHALDEA.
Had the Egyptians and Chaldeans any positive knowledge concerning comets ? —
Apollonius of Myndus ; the Pythagoreans considered comets to be true stars —
According to Aristotle they are transient meteors ; fatal influence of the authority
of this great philosopher upon the development of Cometary Astronomy.
SUCH is a very brief history of the. errors into which the
human mind — we should rather say the human imagination —
has fallen with respect to comets. We have now to show how
little by little, and by very slow degrees, truth disengaged
itself from error, and to supplement the history of superstitions
and prejudices by that of science. Both are instructive and
throw light upon each other at all stages of their mutual
development. Thus, for example, we may readily conceive
that the irregular movements of comets, their sudden and
unforeseen apparition, to say nothing of the singularity of their
aspect, for a long time precluded the idea of their being true
stars, subjected to fixed laws, like the planets. Centuries
of work, observation, and research were required for the
discovery of the true system of the world as far as the sun,
the planets, and the earth were concerned ; but a difficulty of
another kind stood in the way of the discovery of the true
movements and nature of comets, since no trouble was taken
to make exact and continuous observations of them. These
difficulties, which were so great an impediment to science,
35 ' I) 2
THE WOULD OF COMETS.
gave, on the contrary, singular encouragement to the pre-
judices, the superstitions, and the hypotheses which appear so
ridiculous in our day. And, in addition, the predominance of
mystic ideas contributed to deter astronomers from a study
which fell rather within the provinc e of the diviner than the
savant.
It is on this account all the more interesting to see a few
just ideas, a few true conceptions, break through the dark
night of ignorance and superstition. This happened, it is
true, at a time and in countries where philosophy, not yet
obscured by scholastic subtleties, was employed in explaining
facts according to natural hypotheses ; and where, by a bold
and happy intuition, the Pythagorean school guessed without
proving the true system of the world.
Are we to attribute to the Chaldeans and to the ancient
Egyptians the first true conceptions concerning the nature of
comets? That they regarded comets as stars subjected to
regular movements, and not as simple meteors, we may
believe, if it be true that they were in possession of means for
predicting their return. Passages in Diodorus Siculus prove
that the Chaldean and Egyptian astronomers hazarded such
predictions ; but, so far as our means enable us to judge, there
is reason to suppose that these predictions were based upon
particular beliefs, more astrological than astronomical. The
passage which occurs in Diodorus Siculus relative to the
Chaldeans is as follows: —
' The Chaldeans,' says he, ' by a long series of observations
have acquired a superior knowledge of the celestial bodies and
their movements : a knowledge that enables them to announce
future events in the lives of men ; but according to them, five
stars, which they call interpreters, and which others call
planets, deserve particular consideration; their movement is of
singular efficacy. They announce likewise the apparition of
36
COMETS AND THE ASTRONOMERS OF EGYPT AND CHALDEA.
comets, eclipses of the sun and moon, and earthquakes; all
changes that take place in the atmosphere, whether salutary
or pernicious, both to whole nations and kings and simple
individuals.' Diodorus, also, speaking of the astronomical
observations of the Egyptians and their knowledge of the
movements of the celestial bodies, assures us ' that they often
predicted to men what would happen to them in the course of
their lives, the event following the prediction.' ' It is not
unusual,' he adds, ' to hear of them announcing the maladies
which are about to attack men or animals. In short, by means
of accumulated observations, they predict earthquakes, inun-
dations, the births of comets, and, indeed, all that seems to
transcend the limits of the human mind.'
It is clear that, in the opinion of the historian, the pre-
dictions relative to comets which he attributes to the Egyptians
and Chaldeans have no connexion with astronomy. Comets
are confounded with other atmospheric meteors, whose return,
according to them, was connected with the course of the stars
by rare and mysterious coincidences, with which astrologers
had far more to do than astronomers.
Nevertheless, we may suppose that the Chaldeans possessed
some just ideas on the subject of comets. From them, indeed,
and from the Egyptians * the Greeks derived their first know-
ledge of astronomy ; from them, if Seneca is to be trusted,
Apollonius of Myndus obtained his ideas concerning these
stars. According to Apollonius ' comets are placed by the
* ' Eudoxus first brought with him from Egypt into Greece a knowledge
of their movements [the planets]. Nevertheless, he makes no mention of
comets. Hence it follows that even the Egyptians, a people more curious than
any other in all matters of astronomy, had occupied themselves but little with
the study of 'these bodies. At a later period, Conon, a most accurate observer,
drew up a catalogue of the various eclipses of the sun recorded by the Egyptians,
but he makes no mention of comets, which he would hardly have omitted if he
had found any facts respecting them.' — Seneca, Qucestiones Naturales, vii. 3.
37 /
THE WORLD OF COMETS.
Chaldeans amongst the number of wandering stars, and they
know their course.' Seneca then explains in detail the opinion
of this ancient astronomer. ' A comet is not an assemblage of
planets, but many comets are planets. They are not false
appearances, nor fires burning on the confines of two stars;
they are proper stars, like the sun and moon. Their form is
not exactly round, but slender and extended lengthwise.
Moreover, their orbits are not visible ; they traverse the highest
regions of the heavens, and only become apparent at the lowest
part of their course. We are not to suppose that the comet
which appeared under Nero, and removed infamy from comets,
bore resemblance to the comet which, after the murder of
Julius Caesar, during the games of Venus Genetrix, rose above
the horizon about the eleventh hour of the day. Comets are
in great number and of more than one kind ; their dimensions
are unequal, their colours are different ; some are red, without
lustre; others are white, and shine with a pure liquid light;
others again present a flame neither pure nor fine, but enveloped
in much smoky fire. Some are blood -red, sinister presage
of the blood soon to be shed. Their light augments and
decreases like that of other stars, which throw out more light and
appear larger and more luminous in proportion as they descend
and come nearer to us, arid are smaller and less luminous
when they are returning and increasing their distance from us.'
Seneca, as we shall soon see, adopts this system, in which
observations and conjectures nearly approaching the truth are
mixed with various errors and traces of the reigning super-
stitions. The assimilation of comets to the planets as far as
concerns their movements is a luminous idea, which is all the
more truthful because Apollonius points out at the same time
a characteristic difference between the two kinds of celestial
bodies — viz., that comets are only visible in a small portion of
their orbits.
38
COMETS AND THE ASTRONOMERS OF EGYPT AND CHALDEA.
Amongst the ancient philosophers who believed comets to
be stars — stars wandering like the planets — must be mentioned
Diogenes, chief of the Ionic school after Anaxagoras (Plu-
tarch), Hippocrates of Chios, and several Pythagoreans. A
passage in Stobseus, 5th century A.D., proves, as also book vii.
of the Qucestiones Naturales of Seneca, that this opinion of
the ancients concerning the true nature of comets remained
uselessly chronicled in the books which have come down to
us through the Middle Ages. Astronomers derived from it no
benefit, so general was the superstition and so profoundly was
it rooted in all minds. The passage in Stobseus runs : ' The
Chaldeans believed comets to be other planets, stars which are
hidden for a period, because they are too far distant, and which
sometimes appear when they descend towards us, according to
the law prescribed for them ; they consider that they are called
comets by persons ignorant of their being true stars, which
only seem to be annihilated when they return to their own
region and plunge into the profound abyss of ether, as fishes
plunge to the bottom of the sea.'
What was required to render fruitful these remarkable
views? Simply to the observation of comets to apply the rules
long known and followed by astronomers for noting with
precision all the circumstances of the movements of the planets.
How precious would such observations now be to us for
cometary theories! We must admit, however, that to have
extracted from them all that they could yield, it would have
been also requisite to have risen at one bound to the conception
of the true system of the world, dimly seen by the Pythagorean
school, and allowed to repose in the shade till the days of
Copernicus and Galileo.
What were the obstacles which opposed so natural a pro-
gress in science ? First, and most powerful of all, the enslave-
ment of minds to the belief in the supernatural, and the pre-
39
THE WOBLD OF COMETS.
vailing misconceptions on the subject of comets ; prejudices
which increased in strength from the time of the Greek philo-
sophers to the Middle Ages, when astrological folly attained its
maximum intensity. There was at work also the influence of a
powerful genius, who adopted— not very decidedly, it is true—
the erroneous theory of the comet-meteors. In those ages,
when everyone was always ready to swear per verba magistri,
the word of Aristotle sufficed to ensure conviction, and the
ideas of Apollonius of Myndus and of Seneca were regarded
as tainted with heresy.
Pinore' divides the opinions of the ancients about comets
into three principal systems : that which we have just noticed,
and which is, as it were, a rough sketch of the true system ;
that of Panrctius, who regarded comets as destitute of all
reality — a simple optical appearance only ; and, lastly, the
system according to which comets are simple atmospheric
meteors, transient and sublunary. Amongst the authors of
these different systems some, like Heraclides of Pontus and
Xenophanes, regarded comets as very elevated clouds illu-
minated by the sun, the moon, or stars, or even as burning
clouds. Transport these clouds from the atmosphere into
the heavens themselves, into the regions where the planets
perform their revolutions, and we have nearly the opinion of
contemporary astronomers. The same might be said of the
notion of Strato of Lampsacus, who regarded comets as lights
sunk deep in the midst of clouds of great density, thus com-
paring them in some sort to lanterns. Does not the luminous
nucleus in the centre of the nebulosity, which the telescope
of modern times has revealed, correspond, in fact, to the hypo-
thesis of the peripatetic philosopher ?
We now come to the views of Aristotle concerning comets,
views absolutely false, though maintained but two centuries
ago, but yet important, on account of the great influence they
40
COMETS AND THE ASTRONOMERS OF EGYPT AND CHALDEA.
exercised over the astronomers of the Middle Ages, and even
over those of the Renaissance. In the opinion of this great
philosopher comets are exhalations rising from the earth,
which, having reached the upper regions of the air, adjoining
the region of fire,* are drawn along by the movement of the
surrounding medium. They at last unite with it, condense,
and catch fire ; so long as the combustible matter lasts the
fire burns ; when there is no more aliment for its supply the
fire becomes extinct and the comet disappears.
It is useless to refute this hypothesis, which is entirely
without foundation, or to record the objections which have
been made to it by writers even of the time of Seneca. But it
is well to devote a few words to this last philosopher. The
book of the Qucestiones Naturales in which he relates all that
was known in his time of comets, their movements and in-
fluence, is of great historic value, and the views of the author
himself are certainly worthy of attention on their own account.
* According to Aristotle the air is divided into three regions : that in which
animals and plants exist ; this is the lower region, which is immovable, like the
earth upon which it rests ; the intermediate region, intensely cold, participates
in the immobility of the first ; but the upper region, contiguous to the region
of fire or the heavens themselves, is carried along by the diurnal movement of
the latter. The exhalations arising from the earth ascend to this higher region,
and there, heated by the medium they have entered and by their own movement,
they engender igneous meteors to which class comets belong.
41
SECTION II.
COMETARY ASTEONOMY IN THE TIME OF SENECA.
Book vii. of Seneca's Quastmnes Naturales relates to comets — Seneca defends in it the
system of Apollonius of Myndus ; he puts forth just views concerning the nature of
comets and their movements — His predictions respecting future discoveries in regard
to comets — The astronomers of the future.
FROM the beginning of his book Seneca fully appreciates the
importance of the question, and the connexion that must neces-
sarily exist between the nature of the comets and the system of
the universe itself. He is led to ask ' if comets are of the same
nature as bodies placed higher than themselves. They have
points of resemblance with them, ascension and decimation,
and also outward form, if we except the diffusion and the
luminous prolongation; they have likewise the same fire, the
same light.' Here, then, we have comets assimilated to the
planetary bodies as regards their movements, the only points
of difference being the nebulosities and tails of the former,
Seneca is sensible how important it would be * to discover, if
possible, whether the world revolves about the motionless
earth, or if the world is fixed and the earth revolves ; whether
it is not the heavens but our globe which rises and sets.' ' It
would be necessary,' he adds, ' to possess a table of all the
comets which have appeared; for their rarity up to the present
time has been a hindrance to our understanding the laws
o
which regulate their course, and assuring ourselves if their
42
COMETARY ASTRONOMY IN THE TIME OF SENECA.
course is periodical, and if a constant order brings them back
to an appointed day. Now, the observation of these celestial
bodies is of recent date, and has only been introduced very
lately into Greece.' It does not appear that Seneca himself
assisted at all the realisation of this reasonable and intelligent
desire. In his time several comets appeared, but he hardly
mentions them in his book, and relates no circumstance of the
apparitions capable of informing us with any certainty of their
apparent course.
After these preliminary considerations, which indicate so
just a presentiment of the truth in the mind of the Roman
philosopher, he proceeds to the explanation of the principal
systems of his time, conceived for the explanation of comets.
He applies himself to refute the system of Epigenes. who, like
Apollonius of Myndus, had. consulted the astronomers of
Chaldea, but with a very different result, the theory of Epi-
genes being very nearly the same as that of Aristotle, with
the exception of a few details equally false. Seneca, in
combating these views, opposes to them objections that are
sometimes very just, as, for example, when speaking of come-
tary movements : ' There is nothing confused,' he says, ' nor
tumultuous in their behaviour; nothing by which it might be
inferred that they obey elements of disturbance or inconstant
principles. And then, even if whirlwinds should be strong
enough to seize upon the humid exhalations of the earth and
bear them upwards to such heights, they would not rise above
the moon; at the level of the clouds the action would cease.
Now we see that comets move in the highest heavens amongst
the stars.'
Seneca has carefully noted one of the characteristic dif-
ferences between comets and the planets. ' Let us bear in
mind,' he observes, ' that comets do not show themselves in one
region of the heavens alone, nor exclusively in the circle of
43
THE WORLD OF COMETS.
the zodiac. They appear in the east and also in the west, but
most frequently towards the north. The comet has its own
region; it completes its course; it is not extinguished; it
withdraws from our range of sight. If it were a planet, its
path, it will be said, would be in the zodiac. But who can
assign an exclusive limit to the stars, and confine and restrict
these divine beings? The planets themselves, which alone
seem to us to move, describe orbits different from each other.
Why should there not be stars following courses of their own
far removed from the planets ? Why should any region of the
heavens be inaccessible ? '
Further on he explains with sufficient clearness the cause
of the retrogressions observed in the movement of the stars and
comets, and also of their occasional stationary positions.
' Why,' he says, ' do certain stars seem to turn back upon
their journey? It is their meeting with the sun which gives
an appearance of slowness to their movements ; it is the nature
of their orbits and of circles disposed in such manner that
at certain moments there is an optical illusion. Thus, vessels
even when in full sail appear to be immovable.' This is in
effect the true explanation, arid equally applies to the move-
ments of the comets.
Seneca enumerates and describes the varied forms presented
by their aspect, and then affirms that all comets have the same
origin, an opinion altogether arbitrary, and relating to a matter
still undetermined at the present day. Upon many points he
has caught glimpses of the truth, sometimes supporting his
views by reasons dictated by good sense, sometimes maintain-
ing his opinion by explanations which in our day create a smile,
borrowed as they are from the ideas of meteorology, astronomy,
or physics received at that time, ideas quite without value, and
which can only be looked upon as the crude utterances of an
infant science.
44
COMETARY ASTRONOMY IN THE TIME OF SENECA.
He quotes the passage of the historian Ephorus concerning
the comet of B.C. 371, a passage of extreme value, as it
testifies to a phenomenon we have seen repeated in our own
day, viz., the division of a comet into two parts. But it is
only to treat the narrator as a dupe or an impostor. Let us,
however, be just: thirty years ago our astronomers held the
same opinion as Seneca, and Pingre' does not fail in this case to
applaud his discernment. The doubling of Biela's comet under
our own eyes was requisite in order to obtain for the testimony
of Ephorus the authority which Seneca and, after him, so many
modern astronomers had refused to it.
The analysis given by our philosopher of the opinion of
Apollonius of Myndus affords him an opportunity of pro-
nouncing in favour of a system of which the cometary theories
of modern times are the infinitely extended development. But
he is not contented with telling us what seems to him most
probable ; he boldly prophesies in the name of the science of
the future. These passages from the Qucestiones Naturales do
great honour to Seneca, and deserve to be quoted as testimonies
of the power and penetration of his intellect.
1 Why,' he observes, ' should we be surprised that comets,
phenomena so seldom presented to the world, are for us not
yet submitted to fixed laws, and that it is still unknown from
whence come and where remain these bodies whose return
takes place only at immense intervals? Fifteen centuries have
not elapsed since
Greece counted the stars by their names.
How many people, at the present day, know nothing of
the heavens except their aspect, and cannot tell why the moon
is eclipsed and covered with darkness ! We ourselves in this
matter have but lately attained to certainty. An age will
come when that which is mysterious for us will have been
45 /
THE WORLD OF COMETS.
made clear by time and by the accumulated studies of cen-
turies. For such researches the life of one man would not suffice
were it wholly devoted to the examination of the heavens.
How then should it be, when we so unequally divide these few
years between study and vile pleasures ? The time will come
when our descendants will wonder that we were ignorant of
things so simple. Some day there will arise a man who will
demonstrate in what region of the heavens the comets take
their way; why they journey so far apart from other planets,
what their size, their nature. Let us, then, be content with
what is already known; let posterity also have its share of
truth to discover.' *
* [Gibbon makes the following excellent remark (Decline and Fall, ch. xliii.)
' Seneca's seventh book of Natural Questions displays, in the theory of comets, a
philosophic mind. Yet should we not too candidly confound a vague prediction,
a veniet tempus, <J-c., with the merit of real discoveries.' — ED.]
46
SECTION III.
COMETS DURING THE RENAISSANCE AND UP TO THE TIME
OF NEWTON AND HALLEY.
Apian observes that the tails of comets are invariably directed from the sun — -
Observations of Tycho Brahe" ; his views and hypotheses concerning the nature of
comets — Kepler regards them as transient meteors, moving in straight lines
through space — Galileo shares the opinion of Kepler — Systems of Cassini and
Hevelius.
SIXTEEN CENTURIES passed away between the prediction of
Seneca and its full realisation through the accumulated
researches of many astronomers and the publication of the
Principia, in which Newton demonstrated the law of cometary
movements. There is nothing to tell of the history of comets
and of systems during this long and dreary period in which
the doctrine of Aristotle prevailed, except that it is entirely
filled with astrological predictions. Our first chapter contains a
resume of all that the learned have found of interest concerning
the apparition of comets and their formidable signification.
Towards the middle of the sixteenth century the move-
ment of the Renaissance, so favourable to letters and the arts,
extended its beneficent influence to the science of observation,
At the end of the fifteenth century, we find Regiomontanus
describing with care the movements of comets, Apian observ-
ing that cometary tails are always turned in a direction from
the sun; Cardan remarking that comets are situated in a
47
THE WORLD OF COMETS.
region far beyond the moon, founding his opinion upon the
smallness or absence of parallax. The time had arrived when,
instead of proceeding by way of conjecture and hypothesis,
astronomers began to multiply observations and to give them
that character of exactness and precision which they had
hitherto so much needed. Many erroneous hypotheses were
yet to be made, but they were subjected to discussion, and the
geometrical conclusions to which they led were compared with
the facts of observation. Astronomers of high repute like
Tycho Brahe, Kepler, Galileo, Hevelius and Cassini were to
err as to the true nature of cometary orbits; philosophers like
Descartes were to seek to connect them with their bold but
false conceptions of the system of the world. But the great
principle that was destined to bind in one majestic whole the
entire edifice of accumulated astronomical knowledge, the
principle of gravitation, was ere long to give Newton a right
to regard these bodies as members of the solar system, or at
least as bodies subject to the same laws as the planets. From
this moment cometary astronomy begins, and rises rapidly to a
degree of development comparable to that of other branches of
astronomy.
We will first give a rapid sketch of the principal phases of
this history up to the time of Newton, and then proceed to the
study of comets in connexion with their movements, their
physical and chemical constitution, &c.
The apparition of the comet of 1577 may be regarded as
the starting-point of the new period. Tycho, who had carefully
observed the temporary star of 1572, which had suddenly
appeared in Cassiopeia, now applied himself to make numerous
observations of the new comet; he determined its parallax,
and thus proved beyond a doubt that comets move in regions
more remote than the moon, as Cardan had already remarked.
Tycho endeavoured to represent the movement of the comet
48
COMETS DURING THE RENAISSANCE.
by making it describe around the sun an orbit external to
Venus. With respect to its physical nature he regarded it as
a meteor, but not an atmospheric meteor, since he supposed
it to have been engendered in the depths of space. This was
a first blow to the ideas of Aristotle, which other contemporary
astronomers, such as Maestlinus and Rothmann, continued to
profess.
The comets of 1607 and 1618 furnished Kepler with an
opportunity of explaining their apparent movements, and in-
venting an hypothesis which, although false, was ingenious.
According to the immortal author of the three great laws of
the planetary motions, comets traverse the solar system in
rectilinear orbits, and Pingre* justly remarks that the apparent
movement of the comets of 1607 and 1618 is more naturally
explained by this hypothesis than by that of Tycho, which is
equivalent to saying that the paths of the two comets were
more nearly straight lines than circles. As to the physical
nature of comets, believed by Kepler to be as numerous in the
heavens as fishes in the sea, his remarks on the subject taken
from the second book of his work upon comets are as follows :
' They are not eternal, as Seneca imagined; they are formed of
celestial matter. This matter is not always equally pure; it
often collects like a kind of filth, tarnishing the brightness of
the sun and stars. It is necessary that the air should be puri-
fied and discharge itself of this species of filth, and this is
effected by means of an animal or vital faculty inherent in the
substance of the ether itself. This gross matter collects under
a spherical form; it receives and reflects the light of the sun,
and is set in motion like a star. The direct rays of the sun
strike upon it, penetrate its substance, draw away with them
a portion of this matter, and issue thence to form the track of
light which we call the tail of the comet. This action of the
solar rays attenuates the particles which compose the body of
49 • , E
THE WORLD OF COMETS.
the comet. It drives them away; it dissipates them. In this
manner the comet is consumed by breathing out, so to speak,
its own tail.' We see that although, in the opinion of Tycho
and Kepler, comets are raised to the rank of heavenly bodies,
they continue to regard them as stars of temporary origin,
destined to disappear.
Some of the views of Kepler are affected by the singular
and mystic conceptions of the great astronomer concerning the
heavenly bodies; yet those relating to the formation 'of cometary
tails, as we shall see further on, have been perfected and adopted
by contemporary astronomers, and form the starting-point of
one of the most accredited modern theories of cometary
phenomena.
Galileo also believed that comets move in straight lines, but
he was unable to rise above the common opinion, according to
which they were mere transient meteors, exhalations of the
earth.
The remarkable comets which appeared about the middle
of the jixleenth century — namely, those of 1664, 1665, and
1680 — attracted the attention of all men of science; the idea
that they were veritable stars more and more gained ground,
and, after the lapse of fifteen centuries, a definitive return was
made to the system of Apollonius of Myndus; but modern
astronomy was more exacting than the science of the ancient
Greek philosophers. It was necessary to satisfy numerous and
precise observations and to pass beyond vague ideas and con-
jectures. Henceforth the whole question reduced itself to the
investigation of the geometrical form of the orbit described by
comets and to the determination of the laws governing their
movement.
Cassini attacked this great problem, but he did not arrive
at its solution, which is not surprising, when we bear in mind
that this illustrious astronomer did not yet dare to abjure the
50
COMETS DURING THE RENAISSANCE.
beliefs that Copernicus and Galileo had overthrown concerning
the system of the world. By regarding the earth always as a
fixed observatory he could not but confound the apparent
motions of comets with their real motions. Cassini rightly
supposed them to be stars, old as the world, but he made them
describe circular orbits very eccentric to the earth, in order to
Account for the slight portion of the orbit that is visible during
the brief durations of their apparitions.
Hevelius, a laborious observer, came back very nearly to
Kepler's system, that is to say, to rectilinear orbits, or orbits
sensibly rectilinear. Comets, in his opinion also, are the
products of exhalations rising from the earth, the planets, or
the sun. Drawn away at first by an ascensional movement,
combined with the rotatory movement of the planet that has
given it birth, the mass, after having described a spiral, finally
attains the limit of the vortex which surrounds the planet;
there it dies or escapes along the tangent to the limiting
surface. The resistance opposed to it by the ether modi-
fies the form of its orbit, which would otherwise be rec-
tilinear, and causes it to take the form of a parabola. The
whole of this system is purely imaginary, and must have made
great demands upon the imagination of its author ; it rests
upon no solid basis of astronomical mechanics. The ideas of
Hevelius found but few partisans amongst men of science; the
work in which they are developed, valuable for the historic
details it contains, and for various observations of comets,
more especially those of 1652, 1664, and 1665, is little more
than an object of curiosity in the history of science.
Newton, moreover, was about to put an end to all these
hypotheses, by connecting the movements of comets with the
laws that govern the motions of all the heavenly bodies which
move within the sphere of the sun's attraction.
61
SECTION IV.
NEWTON DISCOVERS THE TRUE NATURE OF COMETARY ORBITS-
Newton's Principia and the theory of universal gravitation — Why Kepler did not
apply to comets the laws of the planetary movements — Newton discovers the true
system of cometary orbits — Halley and the comet of 1682; prediction of its
return.
KEPLER, in 1618, had already discovered the three laws upon
which his fame rests, and which will render his name im-
mortal. These laws govern the movements of bodies which,
like the planets and the earth, revolve about the sun in regular
periods. In virtue of the first law the orbit described about
the sun is an ellipse, of which the sun itself occupies one of
the foci ; the second relates to the velocity of the planet, a
velocity which is greater the nearer the planet is to the sun,
and less in proportion as it is further removed; or more
accurately the velocity is such that the areas of the sectors
swept out by the radius vector of the planet are equal in
equal times; hence it follows that the maximum of speed
takes place at the perihelion, and the minimum at the aphelion.
The third law expresses the constant relation which connects
the duration of each periodic revolution with the longest
diameter, or major axis of the orbit.
Why did not Kepler apply the planetary laws to the move-
ments of comets? Why did he leave to Newton the merit of
an extension which now appears so natural? Because those
52
NEWTON DISCOVERS THE TRUE NATURE OF COMETARY ORBITS.
portions of the cometary orbits visible from the earth are
nearly always small fragments only of the immense and elon-
gated curve described by comets in their total revolution ;
because in Kepler's time no instance was known of a comet
having effected its return; and, lastly, because the powerful
mind of Kepler himself was, doubtless, enslaved by the general
belief that comets were passing, transitory meteors.
Newton, aided by the recent progress of mathematical and
physical science, attained to a higher conception of the move-
ments of the celestial bodies ; he discovered the reason of those
laws which the genius of Kepler had extracted from Tycho
Brahe's observations and from his own; he gave them a
mechanical interpretation; in short, he deduced from them the
celestial movements as so many necessary consequences of a
single principle — the mutual gravitation of the masses of these
bodies and the earth.
From that time comets no longer eluded the investigations
of science. Obeying the law of gravitation, describing orbits
like the planets, owning the sun for their common focus, their
movements are distinguished from those of the planetary
bodies chiefly by two important differences, the first of which
arises from the inclination of their orbits to the plane of the
earth's motion : instead of being confined within narrow limits
this inclination may assume any value whatever. From the
earth comets can be seen, and indeed are s'een, in all regions
of the heavens, whilst the apparent paths of the planets are
confined to the narrow zone called the zodiac. The second
difference arises from the fact that a comet generally performs
its revolution in a very elongated ellipse; for this reason we
see only a very restricted portion of its orbit; beyond this arc
of visibility, on either side, the comet is plunged into depths of
space so remote from the earth that it is lost to view. And
then, again, the duration of a comet's revolution is generally
63
THE WORLD OF COMETS.
so great as to render impossible the recognition of the same
comet on two successive apparitions; at any rate, this had been
the case up to the time of Newton. Ellipses so elongated if
we confine ourselves only to the arc described in the neigh-
bourhood of the perihelion, are undistinguishable from parabolas
havino- the same focus and the same vertex. Newton, taking
O
advantage of this approximate assimilation, gave the means of
determining, by the employment of a small number of observa-
tions, the elements of a comet's orbit regarded as a parabola,
a problem much more simple than that which has for its
object the investigation of the complete ellipse.
It still remains to point out another difference between the
motions of comets and the planets. The movements of the
latter are always direct, and invariably take place, for an
observer situated upon the northern side of the plane of the
ecliptic, from left to right, or from west to east. The move-
ment of some comets is direct, and that of others retrograde.
This circumstance had great weight in securing the adoption of
Newton's Primipia in preference to the vortices of Descartes.
If the planetary heavens were filled with vortices of matter
circulating in the same direction around the sun and around
each body belonging to the system, how could we explain the
fact that comets are able to traverse these media in a direction
opposite to that in which the latter are moving?
All these views, so simple, and at the same time so grand
in their entirety, were not, as we know, readily admitted by
the philosophers and astronomers of the time of Newton.
Still imbued with the spirit of systems and sects, some in-
clined to the old doctrines derived from Aristotle, and others
to the bold novelties of Cartesianism.
But the actual truth was very shortly to be made clear.
Halley, an illustrious contemporary of Newton, contributed
to its triumph in the matter of cometary theories. He under-
54
NEWTON DISCOVERS THE TRUE NATURE OF COMETARY ORBITS.
took the calculation — at that time a very laborious task — of the
orbits of twenty-four comets of which the observations appeared
to be sufficiently numerous and accurate. He compared them
with one another, and thought he recognised the identity of
several amongst them. A comet lately observed — that of 1682
— appeared to him similar to the comets of 1607 and 1531.
He satisfied himself of this agreement ; he affirmed it to be the
same comet, observed on several successive apparitions, and
finally predicted its return. Neither Halley nor Newton were
able to see the prediction verified by the event. But the year
1759, when the return of the comet of 1682 did actually take
place, marks an important date in the history of cometary
astronomy, and, from this memorable epoch, there was no
longer room for hypotheses — at all events, so far as the
motions of comets are concerned.
The time has now come for us to enter upon the scientific
portion of our subject.
CHAPTER III.
THE MOTIONS AND ORBITS OF COMETS.
SECTION I.
COMETS PARTICIPATE IN THE DIURNAL MOTION.
COMETS participate in the diurnal motion of the heavens.
During the time of their apparition they rise and set like the
sun, the moon, the stars, and the planets. In this respect,
therefore, they do not differ from other celestial bodies.
Let the observer, when a comet is in sight, note the point in
the heavens which it occupies when his attention is first
directed to it. This is easily done by referring the nucleus,
the brilliant point from which the tail proceeds, to two adja-
cent stars. Let a certain time elapse — an hour, for example;
at the end of that time the three luminous points, the two
stars and the comet, will be found to have changed their
position with respect to the horizon, each having described an
arc of a circle in the heavens. The common centre of these
arcs is the celestial pole, a point situated within a very small
distance of the pole-star ; the lengths of these arcs depend
upon the interval of time between the observations, and the
angular distance of each body from the pole. The direction is
that of the general movement of the heavens and the stars ;
that is to say, from east to west.
We have here, then, a fact which clearly teaches us that a
comet moves in regions beyond the atmosphere of the earth;
for the diurnal motion is an apparent motion, foreign to the
60
THE WORLD OF COMETS.
cornet, and belongs in reality to the observer, or, as we
may say, to the observatory. It is caused by the rotation
of the earth upon its axis. The entire atmosphere of the
earth participates in this movement, and a body immersed
in it — although it might, of course, have a separate motion
of its own — would not participate in the diurnal motion. This
is so elementary a fact that there is no need to insist upon it
further.
The ancients, and even those amongst the moderns who
have regarded comets as meteors of atmospheric origin, have
been compelled either to consider the earth as immovable or
to admit that comets, after being formed within the atmo-
sphere, withdraw from our globe, and, becoming independent,
move in the heavens — a theory, as we have already seen,
adopted by Hevelius.
60
SECTION II.
MOTIONS OF COMETS.
Distinction between comets, nebulse, and temporary stara — Comets, in their motions,
are subject to stationary periods and retrogressions — The apparent complications
arise, as in the case of the planets, from the simultaneous movement of these bodies
and the earth.
THEKE is nothing in the foregoing section to distinguish comets
from the multitude of brilliant stars which nightly illuminate
the azure vault of heaven. Comets, it is true, appear in
regions where before they had not been visible, and after a
time they disappear ; but in this respect they resemble those
remarkable stars which have been seen to shine out suddenly
in the midst of a constellation, to increase in brilliancy for a
time, and afterwards to become faint and disappear; such as
the famous temporary stars of 1572 (the Pilgrim), 1604, 1670,
and 1866, which appeared and became extinct in the constella-
tions of Cassiopeia, Serpens, Vulpecula, and Corona Borealis
respectively. These stars, however, have, without exception,
been distinguished by this peculiarity, that from the first to
the last day of their apparition they continued immovable in
the spot where they first appeared; or, more correctly, that
their only motion was that due to the diurnal revolution of
the heavens. Situated, like the fixed stars, at immense dis-
tances from our system, they had no appreciable movement of
their own during the whole time of their visibility — in some
61
THE WORLD OF COMETS. ,
instances of considerable duration. The same is true of the
nebula, which are distinguished from comets by the fact of
their immobility. Hence comet-seekers have only to pursue a
method analogous to that which astronomers follow for the
discovery of small planets.
Comets, on the contrary, have a motion of their own, a
motion oftentimes of great rapidity ; we can see that they
perceptibly change their places from day to day, and some-
times hour by hour, amongst the constellations. This move-
ment they have in common with
the planets, and it is due, as we
are about to see, to the same
causes.
In the first place, to confine
ourselves to the real movement
of a celestial body and its
gradual change of place in
space. Let us for a moment
suppose the earth at rest. The
Fig. 5.— Proper motion of a Comet ; dis- observer Situated Oil its SUl'faCC
tinetion between a Comet and a Nebula. . ,
•would in that case see the
body in motion gradually overtake and pass the different stars
in its course, and describe upon the concave sphere of the
heavens a curve whose form, position, and apparent dimen-
sions would depend upon the actual path of the body, and
its velocity of motion. For example, the moon, which
describes an oval-shaped curve or ellipse around the earth, in
about a month would appear to describe a great circle in the
heavens from west to east. The planets Mercury and Venus,
which revolve about the sun, and describe closed orbits differ-
ing more or less from a circle, but enclosed by the earth's
orbit, would appear to move from one side to the other of the
central luminary of our system, oscillating periodically to the
G2
MOTIONS OF COMETS.
east and west of it. The superior planets, Mars, Jupiter, and
Saturn, as seen from the earth, would make the tour of the
heavens in unequal periods of time, because these planets
describe orbits exterior to that of the earth, and the actual
time of their revolution depends upon the dimensions of their
orbits.
But this simplicity of motion does not exist for an observer
situated upon the earth, and for the following reasons.
The real and regular motion of the planets becomes com-
bined with the motion of the earth; in the interval of a year
our globe itself moves likewise round the sun in a closed curve
or orbit differing but slightly from a circle; in fact, our
earth moves in an ellipse whose focus is the sun. This
displacement of the earth, it will be readily understood, has
the effect of complicating the apparent motion of the planets ;
that is, their change of position upon the starry vault. Some-
times this motion appears accelerated, as will naturally happen
when the planet and the earth are describing arcs in opposite
directions; the two velocities are then added together, just as
to a traveller in a railway train a second train, moving in the
contrary direction, appears to pass with a speed equal to the
sum of the velocities. But should the two trains be moving
in the same direction, they then separate with a speed equal to
the difference only of their velocities; and if the velocities are
equal, each appears to the other motionless. This is what occurs
in the case of the planets as seen from the earth ; for we
observe that their velocities sometimes decrease and become nil,
in which case the planet is to all appearance stationary among
the stars ; and at other times it appears to retrograde.
Thus these effects admit of a very simple explanation.
They are merely the result of the combination of the respective
movements of the planet and of the earth in their orbits.
Whatever may be the true orbit of a comet in the heavens, its
THE WORLD OF COMETS.
apparent path will always be modified by the continual change
of position of our earth.
In order, then, to determine the orbit of a comet we must
take into account the motion of the earth in its orbit during
the time of the comet's apparition. The stationary periods
and retrogressions — although, as we have seen, admitting of a
most simple explanation — long embarrassed astronomers; but
when the true system of the universe was discovered by
Copernicus, and more fully developed by Kepler, these ap-
parent complications of the celestial movements, which had
always been stumbling-blocks in the way of the erroneous
systems, became so many striking confirmations of the true
theory.
Difficulties analogous in kind, but much more numerous
and grave, long prevented astronomers from discovering the
true nature of comets and the laws which regulate their move-
ments. We shall now see why.
04
SECTION III.
IRREGULARITIES IN THE MOTIONS OF COMETS.
Comets appear in all regions of the heavens — Effects of parallax — Apparent motion of
a comet, in opposition and in perihelion, moving in a direction opposite to the
earth — Hypothetical comet of Lacaille ; calculations of Lacaille and Olbers concern-
ing the maximum relative movement of this hypothetical comet and the earth.
THE orbits which the planets describe about the sun are not
circles, but oval curves, termed ellipses ; these ellipses differ but
little from circles ; that is to say, their eccentricities are small.
Moreover, the planes of the orbits in which they move are
inclined at small angles to the plane of the ecliptic. Hence it
follows that their apparent paths are confined to a compara-
tively narrow zone of the heavens, which zone is called the
zodiac. If we imagine these curves pressed down, as it were, up-
on the ecliptic they will appear as nearly concentric circles de-
scribed about the sun, and so disposed as not to intersect each
other. The distances of the earth and of each of the planets vary
according to the position occupied by these bodies in their re-
spective orbits ; but these variations are confined within very
narrow limits, and hence it follows that the velocities of the
planets change so slightly that the difference is all but imper-
ceptible. The mean diurnal motion of Mercury, which of all
the planets moves the most rapidly, amounts to only 4°5'.
With comets the case is very different. These bodies, as we
fc5 F
THE WORLD OF COMETS.
have seen, are restricted to no region of the starry vault, and
traverse the heavens in all directions, and with very different ve-
locities. The third comet of 1739, and the comet of 1472,
mentioned by Pingre", described in a single day, the first an arc
of 120 degrees— that is to say, the third part of the whole ce-
lestial circumference— the second, an arc of 41 degrees and a
half in longitude and nearly 4 degrees in latitude. Their real
movement was, it is true, in a direction contrary to that of the
earth, so that their apparent velocities were in both cases made
up of the sum of their own and the earth's velocity combined.
Here, then, we have an instance of what is called parallax; that
is to say, the apparent movement of the object is affected by the
observer's own displacement. We might multiply examples of
a similar kind, but the following will suffice. ' The comet of
1729,' says Lalande, ' observed by Cassini during several months,
after advancing more than 15 degrees towards the west from
the head of Equuleus to the constellation Aquila, suddenly
curved round to retrace its path towards the east, thus showing
in a very striking manner the effect of the annual parallax.'
These rapid movements are produced by very simple causes,
the most important of which are the near proximity of the comet to
our globe, and the direction of its movement in relation to that of
the earth. The following is a supposititious case, imagined
by Lacaille, in which the apparent angular velocity of a comet
would be enormous.
This astronomer supposes a comet to be moving in a direction
contrary to that of our globe, and in the plane of the ecliptic ;
it is in perihelion, or at its least distance from the sun, and
consequently at that point of its orbit in which its velocity is
at its maximum. At the same time the earth is supposed to
be in perihelion, and is also moving in its orbit with its greatest
velocity. Lastly, the comet is to be not more distant from the
earth than the moon, and it is to be in opposition.. It is, of
(36
IRREGULARITIES IN THE MOTION OF COMETS.
course, extremely improbable that all these hypotheses should
be realised in the same comet, but there is nothino- im-
o
possible in them. Under these exceptional conditions the
comet, seen from the earth, would describe in the heavens an
arc of nearly 39 degrees in longitude during the first hour, and
of 32 degrees in the hour following. In three hours the total
arc described would amount to 92° 58', and this independently
Fig. 6. — Maximum apparent movement of a Comet and the Earth. '
of the diurnal movement, which would further increase the
velocity by 15 degrees per hour. To an observer situated near
the tropics the comet would ascend from the horizon to the
zenith in less than two hours; it would, however, take a some-
what longer time to perform the second half of its journey and
pass from the zenith to the horizon.
The calculation of Lacaille (modified by Olbers, on account
of an error) is by no means difficult to verify; and there is
67 F 2
THE WORLD OF COMETS.
nothing surprising in the result, if we reflect that the velocity
of each of the two bodies, the comet and the earth, is then
at its maximum; that our globe in one hour at its perihelion
passes over in space a distance nearly equal to nine times its
own diameter (or 67,000 miles/; that the cornet has a velocity
greater than that of the earth, and passes over 94,000 miles in
an hour ; so that, in the direction of their motion, these two
bodies are receding from one another at the rate of 161,000
miles per hour. At the end of a day the comet and the earth
would be more than 3,800,000 miles apart.
It is, therefore, easy to comprehend to what irregularities
of movement comets may be subject. Traversing the heavens
in all directions, in orbits the planes of which cut the orbit
of the earth at every possible inclination, approaching to and
receding from the earth in very short spaces of time, influenced
by the diurnal motion and their own proper motion, in combi-
nation with the earth's displacement, they sometimes suddenly
appear, pursuing a rapid course amongst the stars ; then, to all
appearance they relax their speed, and after coming to a momen-
tary stop reverse their motion, and continue their journey in
an opposite direction, sometimes disappearing at a distance from
the sun, sometimes being drowned in his rays.
It was these movements, these singular appearances, which
so long baffled astronomers, and which the genius of Newton,
guided by a higher conception, finally explained. We will now
proceed to define geometrically the movements and orbits of
comets.
(58
SECTION IV.
THE CEBITS OF COMETS.
Kepler's Laws : ellipses described around the sun; the law of areas — Gravitation, or
weight, the force that maintains the planets in their orbits — The law of universal
gravitation confirmed by the planetary perturbations — Circular, elliptic, and
parabolic velocity explained ; the nature of an orbit depends upon this velocity-
Parabolic elements of a cometary orbit.
WHAT is the nature of a true cometary orbit? In other terms,
what is the geometrical form of curve which a comet describes
in space — what is its velocity — how does this velocity vary—-
and what, in short, are the laws governing the movement of a
comet ?
In order to reply to these questions, and to enable them to
be clearly understood, we must first call to mind a few notions
of simple geometry, and also the principal laws which govern
the motions of the planets.
Kepler, as we have already said, discovered the form of the
planetary orbits, hitherto supposed to be circles more or less
eccentric to the sun. This great man demonstrated that the
form of a planetary orbit is actually an ellipse, that the sun is
at one of the foci of the curve, and that the planet makes its
complete revolutions in equal periods of time, but with variable
velocity ; in fact, that in equal intervals the elliptic sectors
described by the radius vector * directed from the sun to the
planet are of the same area.
* [The straight line joining the sujn to a planet or other body moving under
its action is called a radius vector. — ED.]
60
THE WORLD OF COMETS.
Let us take an example. S being the sun, the closed curve
APB ... is the ellipse described by a planet. The distance of
the planet from the sun is variable, as we see: it attains its
minimum value at A, and its maximum value at B, that is to
say, at one or other extremity of the greatest diameter of the
orbit.
For this reason A is called the perihelion (from rip/, near,
and fcie*, the sun); B is called the aphelion (from a™, from,
and fai*g, the sun). For brevity the radius vector AS is
called the perihelion distance; the radius vector SB the aphe-
lion distance ; and the two united, or the sum of these two
distances, forms the major axis AB of the orbit, Lastly, the
IVnlielie. A \~_
pjg. 7. —Second Law of Kepler. The areas swept out by the radius vector are
proportional to the time.
mean distance of the planet from the sun is exactly equal to
half the major axis.
Let us suppose that the arcs AP, P\P^ and P3B have
been described by the planet in equal spaces of time. Accord-
ing to the second law of Kepler, mentioned above, the three
sectorial areas ASP, PiSP2, and P3SB are equal. If the
curve were a circle, of which the sun occupied the centre, it is
clear that the equality of these areas would involve the equality
of the corresponding arcs; and as the arcs are described by the
planet in equal times, it would necessarily follow that the velo-
city would be the same throughout the entire orbit. In other
words, a circular orbit presupposes an uniform movement.
70
THE ORBITS OF COMETS.
Hut, as a matter of fact, the planets, without exception, describe
around the sun ellipses more or less elongated, that is to say,
orbits differing more or less from a circle. In every case their
velocity is variable ; it is the greatest possible in perihelion ; it
then decreases by imperceptible degrees until the aphelion is
attained, when the minimum of speed takes place. This is a
direct consequence of the second law of Kepler.
A third law, discovered after years of research by this
powerful genius, connects the duration of the planetary revolu-
tions with the length of the major axis of their orbits. This law
we have given elsewhere,* as well as some numerical examples
for making it more intelligible to the non- scientific reader. We
shall not return to it here, but confine ourselves to the remark
that, the time of the revolution of a planet being known, the
dimensions of the major axis of the orbit — that is to say, of twice
the planet's mean distance from the sun — -can be deduced by a
simple calculation.! These laws are not rigorously obeyed by
the planets in their movements. The strictly elliptic motion
* Le Ciel, 4th edition, p. 602.
| [Kepler's third law is that the squares of the periodic times are as the
cubes of the mean distances, that is to say, that if r and R be the mean distances
of two planets from the sun, and t and T be the durations of their revolutions
round the sun, then —
t x t : TxT :\ rxrxr '. ExUxR.
For example, taking the mean distance of th3 earth from the sun as unity, the
mean distance of Venus is 0'7233 ; and the earth performs its revolution round
the sun in 365'26 days, Venus in 224'70 days ; so that, according to Kepler's law,
2247x224-7 : 365'26x365'2G :: 0-7233 x 0-723,3 x 07283 : 1;
or, working out the multiplications indicated,
50,490 : 133,415 :: 0-37845 : 1,
and by division it will be found that each ratio of this proportion is ec^ual
to 2-642.
As another example, suppose there were two planets whose periods of revo-
lution were found to be to one another as 27 to 8, then we should know that
their mean distances were as 9 to 4 ; for
27x27:8x8:: 9x9x9:4x4x 4.— ED.]
71
THE WORLD OF COMETS.
supposes ideal conditions that are not present in nature. But,
by advancing them at an epoch when observations were so far
from accurate, Kepler left it for astronomers and mathema-
ticians coming after him to discover the cause of that mechan-
ism of which he had only been able to detect the general laws.
Huygens, Newton, and later many illustrious mathematicians
(foremost among them Euler, D'Alembert, Clairaut, Lagrange,
and Laplace), have explained the reasons not only for the
general movements of the celestial bodies, but also for all the
irregularities and inequalities which their movements undergo
in the course of time.
Ultimately the whole matter resolves itself into a question
of two causes, or of two forces. One of these forces is none
other than weight or gravitation— the tendency that two bodies
or two stars have to become united, a tendency which is propor-
tional to the product of their masses, and which varies inversely
as the square of the distance that they are apart. By their
weight bodies fall to the surface of the earth when left to them-
selves in the atmosphere. If the force of gravitation alone
existed, the moon would fall upon the earth, and both would
together fall with ever increasing speed into the sun, and so
likewise would the planets arid all the bodies of the solar
system.
But, in addition to the central force of gravitation, each
planet is animated by another force,* which of itself would cause
* [It is perhaps well to explain that this so-called centrifugal force is not a
force in the sense in which gravitation is, i.e., it is not an external force acting
upon the body. If a body were projected in space and were not interfered with
by any external force, it would continue to move in a straight line. In order,
therefore, that it may deviate from a straight line it must be acted upon by some
external influence or force, and the resistance this force woiild have to overcome
for the body to change its direction of motion is called ' centrifugal force. '
Thus the ' centrifugal force' measures the tendency the body has to continue to
move in the direction in which it is moving at thje instant. If then a body
describes a curve, some external force must be continually acting upon it, as it is
72
THE ORBITS OF COMETS.
the planet to escape in a straight line in the direction of a tan-
gent to the point of its orbit occupied by the planet. By com-
bining these two forces, and seeking by geometry and analysis
to determine the actual motion resulting from their simultaneous
and constant action, Newton demonstrated that the laws of this
movement were in conformity with those which Kepler had dis -
covered. If one planet alone existed and circulated around the
sun, and if its mass were inappreciable in comparison with the
enormous mass of that luminary, the elliptic movement would
conform rigorously to Kepler's laws. But the planets are more
than one in number ; they act and react upon each other ; their
dimensions and masses are more or less unequal ; they recede
from and approach one another in the course of their revolu-
tions, and their mutual action upon one another is an incessant
cause of disturbances and perturbations. It is important to
notice that these perturbations are not exceptions in the true
meaning of the word; far from invalidating the theory, they
afford the most striking confirmation of it, since each of these
deviations may be calculated beforehand by the theory of uni-
versal gravitation.
But let us here terminate this necessary digression and
return to the comets.
Newton, as we have seen, by a bold but logical generali-
sation, supposed comets to be subject to the same influences as
the planets, to be borne along by a primitive force of impulsion,
and continually drawn by gravitation towards the sun, the focus
of all the movements of our system. Let us endeavour to ex-
continually changing its direction of motion. In the case of a planet or other
body describing an ellipse round the sun, the sun is continually pulling it towards
itself ; and this continued action is necessary to overcome the centrifugal force,
i.e., to balance its tendency to move at every instant in the tangent to its path ;
in fact, if the action of the sun suddenly ceased, the planet would immediately
move off along the tangent to the ellipse at the point where it was, and with the
velocity it had, at the instant. — ED.]
73
plain by some simple examples what must be the orbit of a
body acted on by such influences ; to explain, let it be under-
stood, not to demonstrate.
Consider, then, a heavy mass, a planet M, gravitating towards
the sun, and at the same time moving with a certain velocity
due to an impulsion foreign to gravitation; and suppose, for
pig. s. — Relation between the velocities and forms of Orbits,
the sake of greater simplicity, that M is situated at the point
where the planet is moving in a direction perpendicular to the
radius vector joining the planet and the sun.
The geometrical form of the orbit described by the planet
about the sun will depend solely upon the relation between
the initial velocity of the planet and the distance of the latter
from the sun. For a certain value determined by this relation
74
THE (WHITS OF COMETS.
the curve described becomes a circle of which the sun occupies
the centre, and the planet traverses with uniform velocity every
part of the circumference. The velocity which for a given
distance compels a mass subject to the law of gravitation to
describe a circle is known as circular velocity. A less
velocity would give rise to an elliptic orbit; in which case, the
sun, instead of occupying the centre of the curve, would be
situated at one of the foci, namely, that which is the further
removed from M\ and the point M would be the aphelion of
the planet.
If the velocity be greater than circular velocity the orbit
would still be an ellipse, having the sun in the focus; but in
this case J/ is the perihelion, and the planet attains its greatest
distance from the focus of attraction at the opposite extremity
of the diameter MS.
The greater the initial velocity the more elongated will be
the orbit, and the greater the eccentricity* of the ellipse. But
if the velocity should attain a certain value — viz., should be
equal to circular velocity multiplied by the number 1*414 (or
by the square root of 2) — at this moment the ellipse, the major
axis of which has been continually lengthening, and has at last
increased in the most rapid manner, changes into a curve with
endless branches, called a parabola. A planet animated by
this velocity, or, let us say, by parabolic velocity, at the moment
when it is at its least distance from the sun — i.e. when it
is at its perihelion — is a body which comes to us out of infinite
* The eccentricity is the distance from the centre of the ellipse to one of its
foci, measured in parts of the semi-major axis, which is taken as unity. In
an elliptic orbit the eccentricity is always less than unity, and is usually expressed
in decimal fractions. Amongst the orbits of the eight principal planets that of
Mercury has the greatest eccentricity. 0'2056 ; Neptune and Venus have the
smallest, 0'0087 and 0'0068. Both these orbits differ very slightly from a
circle. In a parabola the eccentricity is equal to 1. In a hyperbola it is
greater than unity.
75
THE WORLD OF COMETS.
space and returns into infinite space; such a body, supposing
one to exist, before arriving at -that region of the heavens
where the action of the sun preponderates, could form no part
of the solar system. After passing its perihelion it would
depart to an infinite distance; and unless the form of its orbit
should become changed by the disturbing influence of the
planets, it would again become alien to the solar group.
Lastly, to omit no case that can possibly occur, we must
consider that of a planet moving with a velocity greater
than parabolic velocity; the orbit now described will continue
to be a curve of endless branches, but it will be an hyperbola, of
which the sun is, as before, situated at one of the foci.
These preliminary notions understood, we are in a position
to consider the question of the geometrical determination of
cometary orbits.
These orbits are, in general, very long ellipses, of con-
siderable eccentricity, that is, of eccentricity very nearly equal
to unity. And this explains why comets remain visible
during comparatively so short a time, as the arc which they
describe is only a very limited portion of the entire orbit.
During the remainder of their journey they are too far distant
from the earth to be perceived either by the naked eye or by
the aid of the most powerful telescope.
The orbit of a comet being thus a very long ellipse, and
the portion of the arc observed being of very limited extent as
compared with the dimensions of the whole orbit, it follows
that it is generally very difficult to determine to what ellipse
this arc belongs, or even to decide whether it may not form
part of a parabola or hyperbola of the same perihelion
distance.
These different curves are, so to speak, blended into each
other, and only become sensibly distinct at a distance too remote
for the comet to be within our range of vision. In these dif-
76
THE ORBITS OF COMETS.
ferent orbits the positions of the comet obtained by calculation
would not be distinguishable from the positions obtained by
direct observation, or would differ by quantities so small as to
be liable to be confounded with the errors made in the obser-
vations themselves.
Fig. 9. — Cometary Orbits, elliptic, parabolic, and hyperbolic.
This was recognised by Newton, who at once conceived the
idea of simplifying the problem involved in the determination
of cometary orbits. He assumed the orbit, in the first instance,
whatever might be its real form, to be parabolic, because the
elements of a parabola, or the conditions which determine its
position in space, its form, dimensions, &c., are less numerous
and more simple than the elements of an ellipse.
Let us, then, consider what are the elements of a parabolic
orbit. A parabola is a plane curve, that is to say, a curve all
77
THE WORLD OF COMETS.
the points of which are situated in the same plane, which in
our case passes through the centre of the sun. The first
thin* therefore, is to define the true position which this
plane occupies in space. This will be accomplished by deter-
mining first the line of intersection in which it cuts the
plane of the earth's orbit, or the ecliptic ; and, secondly, the
inclination or the angle which the two planes make with one
another.
The comet in its movement necessarily cuts the ecliptic
in two diametrically opposite points, called the two nodes ; the
line which joins these two
points and passes through
the centre of the sun is called
the line of nodes. It is suffi-
cient to know one of the
nodes — for example, the as-
cending node — that is to say,
the node which corresponds
to the passage of the comet
from the region south of the
ecliptic to the region north
of the ecliptic. Let N (fig.
11) be this point, which can
be obtained by calculation
from observations of the co-
met; its position will be de-
termined if we know in de-
grees, minutes, and seconds
Fig. 10.— Confusion of the arcs of Orbits of the ValllC of the arc 0& Ol'
different eccentricities in the neighbourhood of /. i i /-\OTVT J
the perihelion. of the angle OhJy measured
from the zero of the eclip-
tic, in the direction in which the celestial longitudes are
reckoned.
78
THE OK BITS OF COMETS.
This first element is called the longitude of the ascending
node, or, more simply, the longitude of the node. But the
plane of the orbit remains undetermined, unless we add to it
a second element, viz., its inclination.
If, through the centre of the sun, we imagine two straight
lines drawn perpendicularly to the line of the nodes, the one in
the ecliptic, and the other in the plane of the comet's orbit,
these two lines will make between them two angles, the
smaller of which measures the angle between the two planes.
The angle i is the inclination.
It next becomes necessary to define and fix accurately
the actual curve described by the comet in this plane, deter-
mined by the longitude of its' node and its inclination. In the
first place, we must know the position of the planet at its peri-
helion, or least distance from the sun. Let A be this point.
SA is then the axis of the parabola, the direction of which
will be known, if we determine the longitude of the point A,
or of the point TT, obtained by projecting SA upon the ecliptic.
If to the longitude of the perihelion we add another element,
the length SA, or the perihelion distance — which, like all
celestial distances, is measured in parts of the sun's mean
distance from the earth — the vertex of the parabola will be
completely nxed.
The parabolic curve described by the comet is now entirely
defined, both as regards its position in space and its dimen-
sions. It remains, however, to find the direction of the
comet's movement, and to determine at what epoch the
comet will occupy any given position in its orbit. For the
purpose of obtaining the direction we will suppose the para-
bola laid or, pressed down upon that side of the ecliptic where
the inclination is least, or more simply, in the language of
geometers, projected upon the plane of the earth's orbit.
The direction of movement will be called direct, if, when
79
THE WORLD OF COMETS.
estimated from above the ecliptic or from the north region
of the heavens, it takes place in a direction from right to left,
Fig. 11.
Fig. 13. Fig. 14.
Determination of a cometary orbit : parabolic elements.*
or from west to east, as is the case with the earth and
all the planets, and retrograde when it takes place in the
contrary direction.
* 1. Inclination, 20°. Direction in longitude of the line of Nodes, 35° to 215°.
Fig. 11. — Movement retrograde.
Node
Perihelion
35C
. 318C
Fig. 12. — Movement direct.
Node .215°
Perihelion
. 318C
Node
Perihelion
Node
Perihelion
Movement direct.
Movement retrograde.
Fig. 13. — Movement retrograde.
Node 215° | Node
Perihelion . 318° | Perihelion
Fig. 14.— Movement direct.
Node 35° Node
Perihelion . . . 318° Perihelion
80
Movement direct.
Movement retrograde.
35C
112C
215°
112C
215C
112C
35C
112C
THE ORBITS OF COMETS.
Lastly, the exact date of the perihelion passage of the
comet completes the determination of the orbit both in time
and space, so that all other positions are deducible by calcu-
lation from the elements we have mentioned. Figs. 11, 12,
13, and 14 show the different cases that may arise, that is to
say, the different positions the same parabolic orbit may occupy
with respect to the plane of the ecliptic, when the inclination,
the line of the nodes and the perihelion distance remaining the
same, the direction of movement only is varied. It will be
seen that eight distinct paths are open to the comet in space.*
Briefly to recapitulate, we subjoin in the following table
these various elements, in the order usually adopted by as-
tronomers, taking for examples the two great comets which
appeared in 1744 and 1858 : —
T, Epoch of perihelion passage, 1744, March 1. 7h. 55m. 39s. Paris mean time.
TT, longitude of perihelion . . 197° 13' 58" ^
Q. longitude of node . . . 45 47 54 ,, . I^^A'
<,,* r, A-, / Mean equinox, 1744*0
i, inclination . . . . 47 7 41 |
q, perihelion distance . . . 0-222209 J
Movement direct, D.
T, Epoch of perihelion passage, 1858, September 29. 23h. 8m. 51s.
TT, longitude of perihelion . . 36° 12' 31"
Q, longitude of node . . . 165 19 13
i, inclination . . . . 63 1 49
q, perihelion distance . . . 0*57847
Movement retrograde, R.
Such are the elements the determination of which is neces-
sary to enable us to find the orbit of a comet supposed to be
parabolic. These elements are not determined directly, but
* [There are two planes (N A N) each having the same inclination i, and, the
perihelion distance remaining the same, there are therefore four positions of the
vertex (A) of the parabolic orbit, viz., two in each plane, one above and the
other below the plane of the ecliptic, as shown in the four figs. 11-14. As, also,
the direction of motion of the comet in the parabolic orbit may be either direct
or retrograde, we have, in all, eight cases. — ED.]
81 G
THE WORLD OF COMETS.
are derived by mathematical calculation from a certain num-
ber of observations of the comet, at least three accurately
observed positions of the comet being required. Three com-
plete observations are strictly indispensable; but in order to
deduce from them the true curve of the orbit it is necessary
that they should have been made with the utmost precision.
One or two positions of the comet would leave the problem
indeterminate. If we have more than three, they are of great
value for verifying the results given by calculation. Of course
all the observed positions should correspond to points lying on
the orbit which has been determined, or, in other words, the
calculated ephemeris should agree with the apparent path
obtained from direct observations of the comet.
But if, all these considerations being fulfilled, the difference
between the observations and the calculated results should
nevertheless prove too great to be attributed to errors in the
observations themselves, it is then proper to conclude that the
comet is not describing a parabola, and that the hypothesis of
a parabolic orbit must be rejected, in which case there remains
no other alternative than that of a hyperbolic or elliptic orbit.
The latter are much the more common ; and it is thus that we
have been led to recognise the periodicity of certain comets. We
are, in this case, concerned with a body which forms a part of
the solar system, and Avhose movements are regulated in the
.same manner as those of the planets.
82
SECTION Y.
THE ORBITS OF COMETS COMPARED WITH THE ORBITS OF THE
PLANETS.
Differences of inclination, eccentricity, and direction of motion.
IF, then, periodical comets, calculated as such, and known to be
periodical by their return, are governed by the same laws as
the planets, why is a distinction made between these two kinds
of celestial bodies? This is a question of high importance,
and one which we cannot completely answer at the present
moment. A full reply would necessitate some definite know-
ledge concerning the origin of the bodies which compose the
solar world. It would be necessary to have studied and com-
pared the physical constitution of comets with that of planets.
Both in origin and constitution we shall see further on that
they appear to be essentially different. Surveying the ques-
tion, however, from a single point of view, regarding it as a
question of movement only, we can already show differences
which separate these two classes of celestial bodies, and justify
the double denomination by which they are distinguished.
Comets, as we have already seen, appear in any quarter of
the heavens, instead of moving, like the planets, in the narrow
zone of the zodiac. This difference arises from the inclinations
of their orbits to the plane of the ecliptic. Among the prin-
cipal planets Mercury alone has an inclination as great as
83 G 2
THE WORLD OF COMETS.
7 decrees; and among 115 telescopic planets 29 only have
an inclination greater than 10 degrees, and very few exceed
30 degrees;* but we see, on the contrary, the planes of come-
tary orbits admit of all inclinations. Out of 242 comets which
have been catalogued 59 have an inclination included between
0 and 30 degrees, 93 have inclinations between 30 and 60
degrees, and 90 an inclination amounting to between 60 and
90 degrees.
This first characteristic is important. When we add to it .
the second distinction, that, whilst the movement of the planets
is without exception direct, out of 242 comets 123 have a
motion that is retrograde, it is impossible not to recognise
a difference of origin in the two classes of bodies. It is never-
theless curious to remark, that out of nine comets whose return
has been established there are eight whose movement is direct;
one alone, the great comet of Halley, which is a comet of long
period, moves in a direction contrary to that of the planets ;
and one alone, that of Tuttle, a comet of mean period, moves
in a plane whose inclination to the ecliptic is considerable
(54 degrees). The inclinations of each of the eight others are
less than 30 degrees.
Let us proceed to another distinctive feature of cometary
and planetary orbits. We have already seen that of the eight
principal planets Mercury is that which describes an orbit
which differs most from a circle. The distance, however,
between its aphelion and perihelion distances does not amount
to half its mean distance. Its mean velocity is 29-2 miles
per second ; at the aphelion it is not less than 24 '9 miles ; at
the perihelion it attains 37 miles per second. The orbits
of the other principal planets differ much less from the
* Felicitas has an inclination of 31^°, Pallas of 34°. The very great incli-
nations of some of the small planets, belonging to the group comprised between
Jupiter and Mars, have obtained for them the appellation of extra-zodiacal planets.
84
THE WORLD OF COMETS.
figure of a circle. But in the group of small planets there
are orbits the eccentricity of which markedly exceeds the
orbit of Mercury; twenty-six of these ellipses have greater
eccentricities ; but one in particular, that of the planet
Polyhymnia, has an eccentricity comparable to that of some
elliptic cometary orbits. Fig. 15, in which the orbit of Faye's
comet and the orbit of the planet Polyhymnia are represented,
as regards their forms and relative dimensions, clearly shows
how close is sometimes the degree of resemblance in point of
eccentricity between cometary and planetary orbits.
The divergence may be of
any amount; the eccentricity
of the great majority of
cometary orbits is so great
that it may be considered
equal to unity, and this is ex-
pressed, let us repeat, by as-
similating them to parabolas.
Is this assimilation to be
considered absolute, or are
we to suppose that all comets
belong to the solar world?
It appears certain that some
orbits at least are hyperbolic. Fig 15._Comparison of the eccentricities of the
As regards these there can be ^it,of F^e's Comet with that of the Planet
Jrolynymma.
no doubt. But if so, it may
be regarded as not improbable that amongst observed comets
there are some which describe true parabolas; so that, after
having once arrived within the sphere of the solar gravitation,
like those which describe hyperbolic orbits, they take their
leave of us for ever.
Amongst the comets whose periodicity has been calculated
there are some which describe ellipses of such great eccentricity
85
THE WOULD OF COMETS.
that as far as we or our descendants are concerned, it is almost
the same as if they were non-periodic. The great comet of 1769
(eccentricity 0-9992) has a period of about twenty-one centuries ;
at its aphelion it will reach a point in space the distance of which
from the earth will be 327 times the distance of the earth from
the sun. The comets of 1811 and 1680 have periods respec-
tively of 3,065 and 8,814 years (eccentricities 0*9951 and
0-9999). The first cornet of 1780 and that of July 1844 will
only return to their perihelia after journeys the respective
durations of which will be 75,840 years and about a thousand
centuries. These comets will penetrate so far into the depths
of space that at the time of their aphelion they will be distant
from our world about 4,000 times the distance of the sun.
If the calculations upon which these necessarily uncertain
values depend are not rigorously exact, they nevertheless show
that the comets to which they relate always remain an integral
part of our system. Their greatest distance is still fifty times
less than that of the nearest fixed star. The action of the sun
< «viV /A/./ upon these bodies will, therefore, always preponderate over that
of any other body, and their masses will be incessantly drawn
towards those regions of the heavens traversed by our earth,
unless, indeed, the perturbations which the planets can exer-
cise upon them should interfere so as to divert them from
their course and modify the elements of their orbits.
SECTION VI.
DETERMINATION OF THE PARABOLIC ORBIT OF A COMET.
Three observations are necessary for the calculation of a parabolic orbit — Oometary
ephemerides ; what is meant by an ephemeris ; control afforded by the ulterior
observations — Elements of an elliptic orbit — Can the apparition or return of a comet
be predicted ? — State of the question — Refutation by Arago of a current prejudice.
THREE observations of a comet — that is to say, three different
positions (in right ascension and declination) of the nucleus of
a comet, or, in a word, three points of its trajectory or apparent
orbit sufficiently distant from each other — are required, as we
have said, for the calculation of the parabolic elements of the
true orbit.
In the last century this determination was not only a long
and laborious operation, but involved much tentative and
uncertain work. Before engaging in the difficult calculation
of the elements of an orbit, astronomers made trial graphically
and even mechanically of different parabolas, and only began
£he calculation after satisfying themselves that one of these
curves nearly represented the positions furnished by obser-
vation. Great improvements were introduced into these
methods during the last century by Lalande, Laplace, and
Gauss. But the calculation of a cometary orbit is always
a sufficiently complex operation, even if it be simply parabolic,
and it still takes a skilful computer accustomed to this kind
of work, several hours to find approximate values of the
different elements. This is not the place for us, of course,
to attempt an explanation of the work itself.
87
THE WORLD OF COMETS.
A first orbit having been found, what astronomers call an
ephemeris is then deduced from it. This term is applied to the
calculated positions which the comet must have occupied or
will occupy day by day during the period of its visibility.
These calculated positions should agree with the observed
positions, that is to say, with the positions obtained by direct
observations with instruments. This comparison furnishes a
means of control from which it will result either that the
elements are correct, or, on the contrary, that the parabola
is unfitted to explain the movements of the comet. In the
latter case it remains to examine whether this movement might
not be better represented by an hyperbolic orbit, or, as most
frequently happens, by an ellipse. In this way a certain
number of comets have been found to describe ellipses round
the sun, and have been accordingly classed amongst periodical
comets of the solar system.
We may remark, while speaking of elliptic orbits, that
two more elements must be added to the elements of parabolic
orbits for the purpose of determining an elliptic orbit : firstly, the
eccentricity above defined, and v/hich, in conjunction with the
perihelion distance, enables us to calculate the major axis of
the orbit ; secondly, the duration of the revolution, a duration
connected with the value found for the major axis by the third
law of Kepler.*
* Take for example, the following table of the elliptic elements of Tempel's
short period comet, 1867. II., for its return in May 1873 ; e is the eccentricity,
a the semi-axis major : —
Perihelion Passage, May 9-74218.
TT, longitude of perihelion . 238° 2' 34"
£3, longitude of node . . 101 12 50
z, inclination ... 9 12 6
e.
eccentricity ..... 0-5076428
a, semi-axis major .... 3-1721
Movement direct.
Duration of the
revolution,
5 years 97 days.
DETERMINATION OF THE PARABOLIC ORBIT OF A COMET.
This leads us to say a few words on a question which has
nearly always been imperfectly understood by the public,
notwithstanding the repeated explanations of astronomers : we
mean the possibility of predicting the advent of a comet.
Can the apparition of any comet whatever be predicted ?
In these terms the preceding question has been invariably
asked. As regards the public which has faith in astronomical
science, but very little knowledge of astronomy, an answer in
the affirmative is not for a moment doubted ; and, in their
opinion, astronomers who allow themselves to be surprised
by the apparition of a comet have certainly failed in their
duty — in their duty as observers, if the discovery of this new
comet rests with an amateur, and in their duty as mathe-
maticians, if they have not foretold it.
As a rule these reproaches are unjust. They are founded
upon a false idea of the power of astronomy and the
nature of cometary orbits. Arago, who never lost an oppor-
tunity of endeavouring to destroy popular misconceptions on
scientific matters, has given a perfect refutation of this error,
which, nevertheless, is still widely spread. The opportunity
was furnished by the brilliant comet of 1843, which appeared
unexpectedly, its arrival not having been announced by
astronomers, and with reason. Let us, therefore, endeavour,
following the example of the late well-known Secretary of the
Academy of Sciences, to dissipate this generally received error
as far as lies in our power,
On referring to the preceding sections of this chapter we
perceive that the greater number of comets which have been
seen and observed * from the earliest times to the present day
* It should be borne in mind that to see a star and to observe it are two
very different things. In the long list of comets mentioned in history, from the
earliest times to the eighteenth century, when Pingre lived, the indefatigable
author of the Cometographie is unable to find more than sixty-seven comets
observed with sufficient accuracy to allow of their orbits being calculated.
89
THE WORLD OF COMETS.
describe parabolas, or, at all events, ellipses so elongated that
we may be certain either that these comets have never visited
our world before, or that their visits have been made in pre-
historic times. For this reason they will never return, or if
they should return it will be at an epoch so far distant from
our own that it need not for a moment occupy our attention.
It is, therefore, evident that a cornet which thus appears for
the first time within sight of the earth could not have been
announced before it was perceived: no prediction of its appa-
rition was possible.
Here, then, is a first point established, which, I repeat,
applies not only to the great majority of recorded comets, but
also to comets which have been catalogued 5 that is to say, to
comets whose orbits have been calculated with more or less
precision. Out of the 262 comets in the catalogue that we
publish at the end of the present volume nine only are
periodical comets whose return has been verified by observa-
tion ; sixty others have elliptic orbits, but the greater number
of these are so eccentric that for our present purpose they
practically fall within the category of comets with infinite
orbits.
Arago was, then, perfectly justified in the following remarks
in reference to the above question so incessantly repeated by
persons who are not astronomers. ' Is it reasonable to hope,'
said he, 'that a time will come when we shall be able to
predict the arrival within our sphere of vision of comets which
have remained for ages as if lost in the furthest regions of
space, which no one has ever seen, whose action upon the
bodies of the solar system is too small to be appreciable, both
in consequence of the excessive rarity of the vaporous matter
of which they are composed, and of their prodigious distance?
A comet is revealed to man when it becomes visible or pro-
duces some perceptible effect. That which has never been
90
DETERMINATION OF THE PARABOLIC ORBIT OF A COMET.
beheld, and has never produced any observed displacement,*
is for us as if it had never existed. The announcement of
the apparition of a new and totally unknown comet would
belong to the domain of sorcery, and not to that of science.
Astrology itself never pushed its pretensions so far even in the
day of its greatest favour.' — Annuaire de 1844.
* Theoretical astronomy has attained, in fact, to such perfection that the
perturbations of unknown bodies have led to the discovery of new planets, as in
the case of Neptune. Arago, who wrote the above passage in 1844, ten years
before the discovery of the planet Neptune, has thus, as it were, foreshado wed
the possibility of such a prediction. •
91
CHAPTER IV.
PERIODICAL COMETS.
SECTION I.
COMETS WHOSE RETURN HAS BEEN OBSERVED.
How to discover the periodicity of an observed Comet and predict its return — First
method : comparison of the elements of the orbit with those of comets that have
been catalogued — Resemblance or identity of these elements ; presumed period
deduced from it — Second method: direct calculation of elliptic elements — Third
method.
THERE are, however, a certain number of comets of whose re-
turn astronomers are certain, and the time of whose apparition
they can calculate. The prediction of the probable epoch at
which these comets will be situated in regions of the heavens
where they will be visible from the earth, and the determina-
tion of their perihelion passage, can be effected more or less
accurately. These are the comets whose orbits, when calcu-
lated from a sufficient number of observations, prove to be
neither parabolas nor hyperbolas, but, on the contrary, are
closed and elliptic, and such that - the comet thenceforth
continues to describe them in regular periods ; in a word, they
are periodical comets* Newton treated the orbits of comets
as parabolic, merely in order to so represent the arc, always
very short, described in the neighbourhood of the perihelion,
when the" comparatively small distance of the comet from the sun
* [It may be stated here that the duration of revolution of a body, that is,
the time occupied by it in a complete revolution round the sun, is called its
' period.' And, in general, the period of any periodical phenomenon is the in-
terval of time between two of its successive returns to the same position. — ED.]
95
THE WORLD OF COMETS.
renders observations possible. In his opinion comets were bodies
of regular periods, and which described ellipses, certainly very
elongated, but in all respects similar to the planetary orbits.
The first certain proof of the periodicity of a comet, the indis-
putable return of a comet in the same orbit, was, therefore, a
confirmation and a brilliant triumph for the Newtonian theory.
Neither Halley, who had the glory of the first prediction, nor
Newton, who made it possible, lived long enough to see the
event justify the theory. Since then, as we are about to see,
facts of the same kind have been multiplied, and the number
of comets whose return can be calculated, and which, more-
over, have actually reappeared, is already considerable, and is
gradually increasing. Side by side with the planetary system,
therefore, another system was being founded, and the history
of this part of our solar world is sufficiently interesting and
instructive to be given with some detail.
But first let us endeavour to explain by what methods
astronomers discover the periodicity of a comet.
When a new comet, or one supposed to be new, makes its
appearance, can we tell if it has been seen or observed at any
previous epoch? The reply to this question serves as a
foundation to the first method employed for the resolution
of the problem. But the reply is not easy if the apparition
or previous apparitions of the comet (supposing it to have
appeared to us before) have not been observed with some
degree of precision, and if the tradition or record is limited
to a vague mention of the size, the brilliancy of the nucleus,
the form or the dimensions of the tail. The outward ap-
pearance of a comet, its physical aspect, would be in almost all
cases insufficient. We shall see as we proceed that these are
variable features, that the aspect of a comet changes in the
course of a single apparition. But even if it remained the
same, the different circumstances of its visibility and distance
96
COMETS WHOSE RETURN HAS BEEN OBSERVED.
from the earth would suffice to prevent the identification of
the two comets. A comet formerly of extreme brilliancy
might reappear as a feeble nebulosity. It would have been
difficult to recognise the same body in the comet of 1607,
whose light appeared to Kepler pale and weak; in that of
1682, which Lahire and Picard compared to a star of the
second magnitude; in that of 1759, which appeared to Messier
like a star of the first magnitude; and, lastly, in the famous
comet of 1456, ' which all historians (except two Poles),' says
Pingre, ' agree in describing as great, terrible, and of an
extraordinary size, drawing after it a long tail which covered
two celestial signs, or 60 degrees.' These were, nevertheless,
one and the same comet. Astronomers, it is true, mistrust,
and justly, the nearly always exaggerated expressions of the
ancient chroniclers; but precisely for that reason a resemblance
of aspect is not to be relied upon for establishing the identity,
and consequently the periodicity, of two comets. We must
have more precise elements of comparison. These elements
are those of the parabolic orbit, when records have been left
of observations — that is to say, of positions and dates sufficient
for the calculation of the orbit — when, in a word, the comet
instead of having been simply seen has been observed. A
catalogue of ancient comets is therefore necessary, and it was
whilst consulting the table of twenty-four comets which he had
calculated that Halley made the prediction, the history of
which we are about to give.
If the longitudes of the ascending node and of the peri-
helion, the inclination of the plane of the orbit, the perihelion
distance, and the direction of movement, are all the same, or
nearly the same, in two cometary orbits, in all probability we
have two successive, if not consecutive, apparitions of the
same comet. Taking the interval between the apparitions for
the period itself, we are enabled by the third law of Kepler to
97 II
THE WORLD OF COMETS.
calculate the dimensions of the major axis of the corresponding
elliptic orbit, and to assure ourselves that the new orbit is in
accordance with the whole of the known observations. If this
be so, we can calculate more or less exactly the comet's next
return; that is to say, its perihelion passage, and all the
circumstances of its future apparition.
The second method consists in the direct calculation of the
elliptic elements. It requires, as a rule, exact observations,
especially if the orbit be greatly elongated, since there is then
but little difference between the apparent path followed by a
comet, whether it be a parabola, a very long ellipse, or an
hyperbola slightly flattened. The first attempts by this
method — a very legitimate one in theory — prove that it is
subject to many difficulties and uncertainties. Euler, on first
applying it to the comet of 1744, obtained a hyperbolic orbit
from the observations made at Berlin. But afterwards, having
received the observations made by Cassini, he found the orbit
to be a very long ellipse, with a period of many centuries.
The first example of an elliptic orbit calculated with pre-
cision by this second method is, we believe, that of Lexell's
comet (or comet of 1770), a comet of short period (five years
and a half), and having an orbit of comparatively slight elon-
gation, but which, unfortunately — we shall come to its history
further on — has undergone enormous perturbations, and has
not again been seen. Since then the direct calculation of the
elliptic movement, without reference to previous observations,
has been employed for various comets, and with success in
several instances, as the return of the periodical comets of
Faye, Brorsen, d' Arrest, and Winnecke (1819) has been ren-
dered certain by numerous and careful observations.
The above two methods both require observed positions of
the comet, whose periodicity is to be discovered, and also that
these observations should possess a certain degree of accuracy.
COMETS WHOSE RETURN HAS BEEN OBSERVED.
In the absence of these conditions the end may, however, be
attained, but the result is, in that case, as conjectural as the
method itself. This third method consists in making a com-
parison of the different historical comets, in noting the
resemblance of their aspect, and in ascertaining if the intervals
of their successive apparitions agree with the hypothesis of a
certain period, whose duration, in this case, must be neces-
sarily contained nearly an exact number of times in these
intervals. The elements calculated for one apparition may
then suffice to render probable the identity of several comets.
In this way M. Laugier is of opinion that he has identified the
comets of 1299, 1468, and 1799 by assuming a period of one
hundred and sixty-nine years, which is twice included between
the two last dates. In the same manner the comets of 1301,
1152, 760, and several others (which we shall mention pre-
sently) have been identified as former apparitions of Halley's
comet, the true period of which has long been calculated and
known.
99
SECTION II.
HALLEY'S COMET.
Discovery of the identity of the comets of 1682, 1607, and 1531 ; Halley announces
the next return for the year 1758— Olairaut undertakes the calculation of the
disturbing influence exercised by Jupiter and Saturn upon the comet of 1682 ;
collaboration of Lalande and Mdlle. Hortense Lepaute— The return of the comet
to its perihelion is fixed for the middle of April 1759 ; the comet returns on the
13th of March— Eeturn of Halley's comet in 1835 ; calculation of the perturbations
by Damoiseau and Pontecoulant ; progress of theory — The comet will return to its
perihelion in May 1910.
LET us recal the memorable words of Seneca in his Qucestiones
Naturales : ' Why should we be surprised that comets, pheno-
mena so seldom presented to the world, are for us not yet
submitted to fixed laws, and that it is still unknown from
whence come and where remain these bodies, whose return
takes place only at immense intervals'? ... An age will come
when that which is mysterious for us will have been made
clear by time and by the accumulated studies of centuries.
... Some day there will arise a man who will demonstrate
in what region of the heavens the comets take their way,
why they journey so far apart from other planets, what
their size, their nature.' Eighteen centuries have elapsed, and
not one man, but the accumulated efforts of many men have
raised a corner of the veil spoken of by Seneca. As far as the
laws of cometary movement are concerned Newton has realised
his prediction ; whilst that which relates to the return of comets
and their calculated periodicity has been fulfilled by Halley.
100
HALLEY'S COMET.
This learned man, modest as he was laborious, published
in 1705 his catalogue of twenty-four comets. On comparing
their elements he remarked that three comets — namely, those
of 1531, of 1607, and of 1682 —had orbits nearly identical. He
at once suspected the identity of the comets themselves ; and
more than that, he announced the next return of the comet for
the year 1758. Let us subjoin the elements which Halley
calculated, and leave him afterwards to speak for himself:—
Comet of 1531. Cometof 1607. Comet of 1682.
Longitude of node . ' ,' " . 49° 25' 50° 21' 51° 16'
Inclination of orbit . . 17° 56' 17° 2' 17° 56'
Longitude of perihelion . . 301° 39' 302° 16' 302° ,53'
Perihelion distance . . G'56700 0-58680 0-58328
Direction of movement . . Retrograde. Retrograde. Retrograde.
The following is the passage in Halley's memoir * concerning
the periodicity of the comet which at the present day bears his
name : —
' Now, many things lead me to believe that the comet of
the year 1531, observed by Apian, is the same as that which,
in the year 1607, was described by Kepler and Longomon-
tanus, and which 1 saw and observed myself, at its return, in
1682. All the elements agree, except that there is an in-
equality in the times of revolution ; but this is not so great that
it cannot be attributed to physical causes. For example, the
motion of Saturn is so disturbed by the other planets, and
especially by Jupiter, that his periodic time is uncertain, to
the extent of several days. How much more liable to such
perturbations is a comet which recedes to a distance nearly
four times greater than Saturn, and a slight increase in whose
velocity could change its orbit from an ellipse to a parabola ?
The identity of these comets is confirmed by the fact that in
* [The title of Halley's memoir is Astronomies Cometicce Synopsis, and it was
published in the Philosophical Transactions, vol. xxiv. (1704-5), pp. 1882-
1899.— ED.]
101
THE WORLD OF COMETS.
the summer of the year 1456 a comet was seen, which passed
in a retrograde direction between the earth and the sun, in
nearly the same manner; and although it was not observed
astronomically, yet, from its period and path, I infer that it
was the same comet as that of the years 1531, 1607, and 1682.
I may, therefore, with confidence predict its return in the
year 1 758. If this prediction be fulfilled, there is no reason to
d^ubt that the other comets will return.'
Later, in his Astronomical Tables, published in 17-19, ten
years before the return of the comet, Halley recurs again to
his prediction in the most decided terms. ' You see, therefore,'
he says, ' an agreement of all the elements in these three,
which would be next to a miracle if they were three different
comets; or, if it was not the approach of the same comet to-
wards the sun and earth in three different revolutions, in an
ellipsis around them. Wherefore, if according to what we
have already said, it should return again about the year 1758,
candid posterity will not refuse to acknowledge that this was
first discovered by an Englishman.' *
Posterity has remembered and science recognised the claim
of the English astronomer, by giving his name to the first
comet whose periodical return, announced beforehand, was
confirmed by the event. But the same posterity will not be
unjust: it will give a legitimate share of honour to the French
astronomers Clairaut and Lalande, who completed the work of
Halley by calculating the retardation the comet would be
subjected to in its voyage of seventy-six years. This second
part of the history of a great discovery is perhaps still more
surprising and instructive than the first.
As the epoch of the return predicted by Halley drew near,
* [Halley died on January 14, ] 741-2, and his Tabitlte Aatronomicce were
published seven years after his death, in 1749. In 1752 a second edition appeared,
and to it was appended an English translation, from which the passage cited in
the text is extracted. — ED.]
102
HALLEY'S COMET.
all astronomers in France and Europe, occupied with this
great event in the annals of science, held themselves in readi-
ness to make observations of the comet. The time of its
reappearance was uncertain. The known periods, as Halley
had himself remarked, were unequal. Between 1531 and 1607
the interval was 27,811 days; from 1607 to 1682, 27,352
days, with a difference of 459 days between the perihelion
passages. Would the new period be still shorter, or, on the
contrary, after having been diminished by fifteen months and
a half, would it return to its old value, or even exceed it?
Several savants made calculations and offered various hypo-
theses respecting the path of the comet on its return and the
date of its apparition, which was watched for from 1757.
It was then that Clairaut, a great mathematician, under-
took the rigorous solution of the problem which Halley had
only indicated — viz., the calculation of the perturbations which
the comet of 1682 would experience whilst passing in the
vicinity of the planets, especially of Jupiter and Saturn. It
was a work of immense difficulty, and Clairaut, pressed for
time, sought the assistnnce of Lalande. one of the most illus-
trious of French astronomers. Mdlle. Hortense Lepaute, the
lady who has given her name to the Uortensia, undertook
part of this laborious work. Thanks to the devotion to science
of these three worthy collaborateurs, the work was brought to
a close in November 1758, and Clairaut presented to the
Academy of Sciences a memoir from which the following is a
short extract : —
' The comet which has been expected for more than a year
has become the subject of a curiosity much more lively than
that which the public generally bestows upon questions of
astronomy. True lovers of science desire its return because it
would afford striking confirmation of a system in favour of
which nearly all phenomena furnish conclusive evidence.
103
THE WORLD OF COMETS.
Those, on the contrary, who would like to see the philosophers
embarrassed and at fault hope that it will not return, and that
the discoveries of Newton and his partisans may prove to be
on a level with the hypotheses which are purely the result of
imagination. Several people of this class are already triumph-
ing,&and consider the delay of a year, which is due entirely
to°announcements destitute of all foundation, sufficient reason
for condemning the Newtonians.
' I here undertake to show that this delay, far from invali-
datino- the system of universal gravitation, is a necessary
consequence arising from it ; that it will continue yet longer,
and I endeavour to assign its limit.'
Let us say at once that Clairaut found that the perihelion pas-
sage of the comet would be delayed 6 18 days, and that it would
take place in 1759, a hundred days being due to the action of
Saturn, and 518 days to that of Jupiter, bringing the peri-
helion passage to the middle of the month of April. Never-
theless, he made reservations with a modesty not less to his
honour than his immense work, reservations necessitated by
the terms omitted from the calculations, such as unknown
causes of perturbation, and the fear that some errors might
have been committed in the numerous and delicate operations
performed. All these accumulated uncertainties might, ac-
cording to Clairaut, make the difference of a month in the
appointed time. The comet was actually seen on the 25th
of December, 1758, by a Saxon peasant of the name of Palitsh
in the environs of Dresden. Observations were made of the
comet, and astronomers were soon able to prove that the
perihelion passage would take place on the 13th of March,
1759, thirty-two days before the epoch calculated by Clairaut.
Such a triumphant success of the theory produced in the
scientific world a deep impression, and Lalande said with very
legitimate enthusiasm: —
104
HALLEY'S COMET.
' The universe beholds this year the most satisfactory
phenomenon ever presented to us by astronomy; an event
which, unique until this day, changes our doubts to certainty,
and our hypotheses to demonstration. . . . M. Clairaut asked
one month's grace for the theory; the month's grace was just
sufficient, and the comet has appeared, after a period of 586
days longer than the previous time of revolution, and thirty-
two days before the time fixed ; but what are thirty-two days
to an interval of more than 150 years, during only one two-
hundredth part of which observations were made, the comet
being out of sight all the rest of the time? What are thirty-
two days for all the other attractions of the solar system which
have not been included; for all the comets, the situation and
masses of which are unknown to us ; for the resistance of the
ethereal medium, which we are unable even to estimate, and
for all those quantities which of necessity have been neglected
in the approximations of the calculation ? . . . A difference of
586 days between the revolutions of the same comet, a
difference produced by the disturbing action of Jupiter and
Saturn, affords a more striking demonstration of the great
principle of attraction than we could have dared to hope for,
and places this law amongst the number of the fundamental
truths of physics, the reality of which it is no more possible to
doubt than the existence of the bodies which produce it.'
Another return of Halley's comet took place in 1835. It
furnished an opportunity of testing the progress made by
theoretical astronomy during the period of seventy-six years
occupied by the comet in once more performing its revolution.
Taking the perihelion passage of 1759 as the point of de-
parture, and following in the steps of Clairaut, two French as-
tronomers, Damoiseau and Ponte'coulant, independently under-
took the laborious task of determining the epoch of the perihelion
passage of the comet, taking into account the perturbating
105
THE WORLD OF COMETS.
action of the planets. Amongst the unknown disturbing
causes which Clairaut had been unable to take into account, but
which entered into the researches of the two above-mentioned
savants, was the planet Uranus, discovered by Sir William
Herschel in 1781. According to Damoiseau the comet should
have passed its perihelion on November 4 ; according to
PonfcScoulant not till November 13, 1835. Two other astro-
nomers, Lehmann and Rosenberger, had fixed the. dates of
November 11 and 26. On August 5 the comet was seen at
Kome. Observations gave for the exact date of the perihelion
Fig. 16. — Halley's Comet in 1835. 1. As seen by the naked eye October 24. 2. As seen
in the telescope the same clay,
passage November 16, at half-past ten in the morning, the
difference between the observed date and the mean of the
calculated dates being less than three days* The result showed
an increase of sixty-nine days above the length of the preced-
* On re-computing the disturbing influence of the planets Pontecoulant
calculated that a period of 28,006§ days should have elapsed between the peri-
helion passages of 1835 and 1759. Observations proved it to be 20,006
days. The difference, which is only two-thirds of a day, shows what progress had
been made both in theoretical and practical astronomy.
106
HALLEY'S COMET.
ing period, the new period amounting to 27,937 days ; this
increase arose from two antagonistic causes —
1. An increase of 135 days, 34 being due to the action of
Jupiter.
2. A diminution of 66 days, 30 being due to the action of
Saturn, Uranus, and of the Earth.
The duration of this last period was found equal to seventy-
six years and 135 days, or seventy-six years and four months
and a half. An equal period would bring the next time of
perihelion passage to March 29, 1912. But this date will be sub-
ject to modification by the perturbations incident to the journey.
Jupiter will exercise a considerable retarding influence, and
the revolution which the comet is now accomplishing will be
shorter than any yet observed — it will be 27,217 days; that is
to say, hardly seventy-four years and six months. This brings
the next apparition, according to the calculations of Ponte-
coulant, to May 24, 1910, about nine o'clock in the morning.*
If, on the contrary, we look back into the past and consult old
chronicles and records, we shall find several apparitions of
Halley's comet, the dates of which are as follows, some nearly
certain, others somewhat doubtful : —
June 8, 1456. Halley had already given notice of this apparition.
November 9, 1378.
In December 1301. According to the researches of E. Biot and Laugier.
In September 1152. According to the researches of E. Biot and Laugier.
In May 1066.
In September 989.
In June 760. According to the researches of E. Biot and Laugier.
In October 68 t. According to Hind.
In July -J51. According to the researches of E. Biot and Laugier.
In March 141.
In January 66.
In October 12 B.C.
* The elomorits of the orbit calculated for this epoch by the same mathema-
tician give the 16th of May, 1910, about II P.M.
107
THE WOELD QF COMETS.
In addition to these dates Mr. Hind gives the following as
corresponding probably to former apparitions of the same
comet: 1223, 912, 837, 608, 530, 373, 295, 218.
The period which these apparitions lead us to infer (notably
those of 1456, 1378, 760, 451) amounts to about seventy-seven
years and a quarter, which is longer in a marked degree than
that of the three or four last revolutions.* M. Laugier asks if
this diminution which we are obliged to admit is not due
to the same cause as that which has been assigned to account
for the similar diminution undergone by Encke's comet; that
is to say, the resistance of the ether ; or if, as Bessel thought,
it was due to a dispersion or loss of matter abandoned by the
comet in the course of its successive revolutions. These are
questions of high interest, and we shall recur to them again.
* From the year 12 B.C. to the year 1835, 1,847 years have elapsed, a period
comprising twenty-four revolutions of Halley's comet. The mean duration
would thus be 70 years 350 days.
108
SECTION III.
ENCKE'S COMET ; OB, THE SHORT PERIOD COMET.
Discovery of the identity and periodicity of the comets of 1818, 1805, 1795, and 1786 ;
Arago and Olbera — Encke calculates the ellipse described by the comet — Dates of
twenty returns up to 1873 — Successive diminution of the period of Encke's comet.
IN 1818 Ports, one of the most indefatigable of observers and
comet-seekers, discovered at Marseilles a comet which passed
its perihelion on the 27th of the following January. The
elements of the new cornet, when compared with those of
comets already catalogued, gave reason to suppose that
it had been observed in 1805. Arago remarked this to
the Board of Longitude when Bouvard presented the para-
bolic elements of the new comet ; and Gibers, on making the
same remark in Germany, added that it was doubtless the
same comet which had been observed in 1795 and 1786. We
subjoin the elements of the comets of 1818 and 1805:
1818. 1805.
Longitude of perihelion .... 144° 15' 147° 51'
Longitude of node 329° 5' 340° 11'
Inclination . ! . . . . 14° 48' 15° 36'
Perihelion distance ..... G'353 0'378
Direction of movement .... Direct. Direct.
The resemblance was too striking not to produce attempts
to determine the periodicity of the new comet. The elliptic
elements of the orbit were calculated by Encke, astronomer
at the Observatory of Gotha, and for this reason the comet
received the name of Encke's comet ; but it is also sometimes
called the short-period comet, in contradistinction to Halley's
109
THE WORLD OF COMETS.
comet, whose period of revolution is so much longer. The
comet of Encke, in fact, describes its orbit in about 1,210 days,
or three years 114 days. ' If we only consider,' says Poisson,
' the rapidity of its successive revolutions, this body might be
regarded as a planet, but it continues to have a place assigned
to it amongst comets, because of the appearances which it presents,
and because it is not visible to us throughout its entire orbit: In
fact, at the time when Poisson wrote his report the belief in
the extreme elongation of all cometary orbits still existed, and
it seemed improbable that a comet should have so short a
period of revolution. But
successive observations of
its return removed all
doubt, and soon new perio-
dical comets were dis-
covered, which justified the
possibility of cometary or-
bits, comparable in point of
their relatively slight eccen-
tricity with the orbits of
Fie. 17. — Encke's Comet, at its pass.nge in 1838, ,11 , i i
August 13. the planets themselves.
The first return of the short-period cornet to its perihelion
took place towards the end of May 1822. Encke calculated
the epoch of its return, and computed an ephemeris; then,
taking into account the perturbations which must have been
experienced by the 'cornet in the course of its preceding revo-
lution, owing to its passage near to Jupiter, he showed that its
period would be lengthened about nine days, and that the
comet would be invisible in Europe ; and in fact it was only
observed in the southern hemisphere (in Australia).
We extract from the Annuaire du Bureau des Longitudes
for 1872 the epochs of the perihelion passages of the comet from
its discovery in 1818 up to its last and next passage : —
110
ENCKE'S COMET; OK, THE SHORT-PERIOD COMET.
January 27 .
May 23 .
September 16 .
January 10 .
May
August
I'ecember 19
April 12
August 10
4
26
1819
1822
1825
1829
1832
1835
1838
1842
1845
November 26
March 1 5
July 1
October 18
February 6
May 28
September 14
December 29
April 12
1848
1852
1855
1858
1862
1865
1868
1871
1875
By adding to the preceding the previous apparitions of
1786, 1795, and 1805 we have in all twenty observed returns ;
but since the first date twenty-seven consecutive revolutions
have really taken place. Now, if the exact intervals between
the perihelion passages be noted, and the durations of the corre-
sponding periods deduced from them, .we have the following
table, which was calculated by Encke: —
From 1786 to 1795, three times .
1795 „ 1805 „ „
1805 , 1819 four „
1819
1822
1825
1829
, 1822
, 1825
, 1829
, 1832
D.
H.
M.
1212
15
7
1212
12
0
1212
0
29
1211
15
50
1211
13
12
1211
10
34
1211
7
41
1211
5
17
1211
2
38
1210
23
31
1210
21
7
1210
18
29
1210
17
2
1210
11
17
1210
13
41
1832 „ 1835 ....
1835 „ 1838 ....
1838 „ 1842 ....
1842 „ 1845 ....
1845 „ 1848 ....
1848 „ 1852 . . .
1852 „ 1855 ....
1855 „ 1858 ....
The above list testifies to a fact of the highest importance :
the period of the comet is continually diminishing. Will it
continue always to diminish ; and if so, what law does this con-
tinual alteration of the orbit follow? A diminution in the
periodical time of a body cannot take place, according to the
laws of Kepler, without a corresponding diminution in the
length of the major axis, or mean distance of the comet from
ill
THE WORLD OF COMETS.
the sun. Are we, then, to suppose that this comet is continually
approaching the focus of our world, and some day will be pre-
cipitated upon its mass? These interesting questions and this
hypothesis have been studied from different points of view by
several astronomers, who have endeavoured to find the physical
cause of this diminution. We shall return to this point fur-
ther on; it is one that concerns the whole solar world.
112
SECTION IV.
BIELA'S OR GAMBART'S COMET.
History of its discovery ; its identification with the comet of 1805 — Calculation of its
elliptic elements by Gambart— Apparitions previous to 1826 — Peculiarities in the
apparitions of 1832, 1846, and 1872.
SEVEN YEARS elapsed between the discovery of Encke's comet
and that of Biela or Gambart, which likewise may be called
a comet of short period, since it performs its revolution in less
than seven years.
The comet was first observed by an Austrian major of the
name of Biela, at JohaHnisberg, February 27, 1826 ; it was
seen ten days after at Marseilles by the French astronomer
Gambart. The latter, after having calculated the elements of
the parabolic orbit, immediately recognised their resemblance
to those of a comet which had been observed in 1805 and in
1772. The following table affords a comparison between the
elements of these three orbits : —
1772. 1805. 1826.
Longitude of perihelion . . 108° 6' 109° 23' 104° 20'
Longitude of ascending node . 252° 43' 250° 33' 247° 54'
Inclination . . . . 19° 0' 16° 31' 14° 39'
Perihelion distance ., . 1'018 0-89 095
Direction of movement . . Direct. Direct. Direct.
I especially wish to direct attention to these comparisons as
examples of the employment of the most simple method for
determining the periodical orbit of a comet, a method merely
113 I
THE WORLD OF COMETS.
suo-gestive and provisional, and for which direct calculations
are immediately substituted. These calculations for the comet
of 1826 were performed by Gambart and Clausen,* who both
obtained accordant results, and assigned to the duration of the
comet's revolution a period of six years and three quarters.
Damoiseau, then, taking account of the perturbations, was able
to predict its next return, which he fixed for the 27th of Novem-
ber, 1832. The comet made its appearance on the 26th, only
* It is customary to give to a periodical comet the name, not of the observer
by whom it was first seen or observed, but that of the astronomer by whom
the elliptic character of its orbit was first recognised. This cornet that astronomers
of both hemispheres persist in calling the comet of Biela ought, therefore, to bear
the name of Gambart. It is not the only instance of injustice in the history of
astronomy. Non periodical comets generally receive the name of their first
observers — thus we speak of Donati's comet, Coggia's comet, &c. But in our,
opinion the best system of denomination is that of designating ^comets by the
year in which they have effected their perihelion passage, and affixing to them a
numeral, according to the order of their discovery. Thus, we say comet L,
1858 ; comet II., 1858, &c. This method leaves no opening for small rivalries
of the kind above alluded to.
fit seems natural, and is, in fact, unavoidable, that a comet should be known
by the name of the astronomer with whom it is chiefly associated, whether as
calculator or observer, without there being any fixed rule in the matter.
Astronomers attach a particular name to a comet, not with the view of honour-
ing the individual, but of having a convenient name for the comet ; and although
the system of quoting the year and the number is admirable for the majority of
comets, still in the case of those that have become celebrated and are frequently
referred to, some more distinctive and easily remembered appellation is needed.
But in the present case it seems a matter of justice that the comet should be
named after Biela, who not only first discovered it, but who calculated its para-
bolic elements, remarked their similarity to those of the comets of 1772 and
1805, and thence concluded that the orbit was elliptic, and that the period was
six years and nine months. This Biela communicated to the Astronomische
Nachrichten, in a letter dated March 23, 1826. Gaiibart also calculated the
parabolic elements of the comet, and remarked their resemblance to those of the
comets of 1772 and 1805. His letter was dated March 22, and both appear in
the same number of the Astronomische Nachrichten. Thus Biela and Gambart
independently recognised the elliptic motion of the comet, while Biela was in
in addition the first discoverer.
If we adopt the rule that a comet should be named after the astronomer who
first recognised its periodicity, it is clear that Faye's comet — the subject of the
next section — should be named after Goldschmidt. — ED.]
114
BIELA'S OR GAMBART'S COMET.
one day earlier than the date assigned. Thus was perfected
more and more the theory of cometjry orbits based upon the
principle of universal gravitation.
Including the previous apparitions of 1772 and 1805, the
comet of six years and three-quarters has been observed on
seven of its returns — in 1826, in 1832, in 1846, in 1852, arid
in 1872. It should have been observed in 1839, 1859, and
1866. 'In 1839,' says Mi Delaunay, 'it could not be observed
on account of the unfavourable position of its orbit at the time
of its perihelion passage.' This passage, in fact, took place
during the first days of July, and both before and after the
comet was situated in close proximity to the sun, and conse-
quently lost sight of in his rays. Nearly the same thing
happened in 1859, the perihelion passage taking place in the
first days of June. Lastly, in 1866, although the cornet
could not have been far distant from the earth about the
time of its perihelion (the 26th of January), and notwith-
standing the diligent search made for it with powerful instru-
ments, it was not discovered. It was last seen at Madras
by Mr. Pogson, on the 2nd and 3rd of December, 1872.
Gambart's comet has furnished some curious events
in the history of physical astronomy. In the beginning
of 1846 the comet divided into two distinct comets, which
appear at the present day in the catalogues, with their respec-
tive orbits. Moreover, in 1832, like the comet of 1773, it
had the privilege of exciting fears which at that epoch
were certainly without foundation. The cornet was to come
into collision with the earth There was more reason to
believe in the possibility of such an encounter at the end
of November 1872; and if it is not one of the twin comets
that then just grazed the earth, it is at least one of their
fragments. I here restrict myself to the simple mention of
these events, which further on will receive the development
they merit.
115 1 2
SECTION V.
FAYE'S COMET.
First comet whose periodicity, without comparison with previous dates, has been
determined by calculation and verified by observation — M. Le Verrier demonstrates
that it has nothing in common with the comet of Lexell — Slight eccentricity of
Faye's comet and great perihelion distance— Dates of its return — Perturbations in
the movements of Faye's comet inexplicable by gravitation alone : a problem to,
be solved.
A COMMUNICATION by Arago, published in 1844, in the Comptes
Eendus of the Academy of Sciences, gives an account of the
first researches relative to the fourth periodical comet, which
we here subjoin : —
' This body was discovered at the Observatory of Paris
by M. Faye, on November 22, 1843. This young astronomer
hastened to calculate its parabolic elements. But as the
number of observations increased M. Faye perceived that a
parabola was quite inadequate to represent the series of posi-
tions occupied by the comet, and announced that he should
determine its elliptic orbit, as soon as the state of the sky
should have permitted him to pursue his observations of the new
comet in regions so far removed from those in which it had
first appeared that no doubt could possibly exist as to the
certainty of his results. M. Faye therefore applied himself to
the multiplying of observations, which had become extremely
difficult to obtain, on account of the indistinctness of the
comet. Matters were in this stage when a letter from
lie
FAYE'S COMET.
Schumacher informed him that Dr. Goldschmiclt, a pupil of
Gauss, had already calculated an elliptic orbit, having used the
observations made at Paris on November 24, and those of De-
cember 1 and 9, made at Altona.'
Here, then, is a comet whose periodicity has been at once
determined by calculation, and without comparison with the
elements of comets previously observed. As the period of its
revolution is short, less than seven years and a half (seven years
151 days), it was thought remarkable that the cornet had not
been seen before. But as at its aphelion it had probably passed
within close proximity to the orbit of Jupiter, it was supposed
that its orbit had been altered by ' that most powerful member
of the solar system,' and that ' from parabolic it had become
elliptic and periodic.' Hut in reality, according to the calcu-
lations of M. Goldschmidt, Faye's comet could not have
been within sixty millions of miles of Jupiter, and this
hypothesis had therefore to be abandoned. M. Valz even
thought to identify the new comet with the famous missing
comet of Lexell, or that of 1770. But M. Le Verrier showed
that there was nothing in common between the two cornets.
The absence of previous observations appears, therefore, to
prove no more than that at the time when its former apparition
should have taken place the comet was not favourably situated
for observation, a hypothesis which can easily be accepted in
the case of a body so faint as to be visible only in the telescope.
Jts orbit presents two rather remarkable peculiarities. In
the first place, it is the least eccentric of known cornetary orbits,
as we have already had occasion to remark when calling atten-
tion to the ellipses described by it and the small planet Polyhymnia
(tig. 15). Besides this, its perihelion distance is somewhat
considerable (1-682). When passing the point of its orbit
nearest to the sun the comet is twenty-seven millions of miles
further removed than Mars at his perihelion, and twenty-two
117
THE WORLD OF COMETS.
millions of miles further than Mars at his mean distance. At
its aphelion it is beyond the orbit of Jupiter— as is the case
with all the other periodical comets, with the exception of
Encke's— exceeding it by more than half the distance of the
sun from the earth, or by fifty millions of miles. What chiefly
caused the great perihelion distance of Faye's comet to be
remarked is that, in the same year, the great comet of 1 843
approached the sun to within 650,000 miles of its centre, so
that the nebulous nucleus was not more than 210,000 miles
from the surface of the solar photosphere — less than the dis-
tance of the earth from the moon.
Faye's comet returned to its perihelion on April 2, 1851, at
two o'clock in the morning, about a day after the epoch that
M. Le Verrier had calculated for the time of its passage, taking
into account the perturbations it had been subjected to from
the disturbing influence of Jupiter. It was seen again in 1858,
again in August 1865, and lastly in September 1873. The
period of its revolution is 7 years 151 days, or 2,708 days ; that
is to say, 323 days longer than that of the planet Sylvia, the
most distant from the sun of the small planets circulating be-
tween Mars and Jupiter.
On carefully studying the three first successive apparitions
of Faye's comet M. Axel Moller detected variations in the
orbit impossible to be explained by gravitation alone. There
is need, therefore, as with Encke's comet, for enquiry into the
cause of this perturbation. This interesting question will be
treated in one of the later chapters which we shall devote to
the hypotheses which have been suggested to account for such
anomalies.
118
SECTION VI.
BRORSEN'S COMET.
Discovery of the comet of five years and a half period by Brorsen in 1846 — Its
supposed identity with the comet of 1632 gives reason to suspect elliptic elements ;
calculation of these elements — Returns of the comet in 1851, 1868, and 1873.
IN the order of their discovery we proceed to pass in review
the periodical comets of the solar system — those at least whose
return has been confirmed by observation, and which have
justified the predictions of calculation.
This brings us to a comet which likewise bears the name
of the astronomer who discovered it, at Kiel, on February 26,
1846, viz. to Brorsen's comet, whose period is intermediate to
those of Encke and Faye. It performs its revolution round
the sun in five years and a half, or, more exactly, in five years
176 days, or 2,002 days.
As soon as the parabolic elements of the new comet were
calculated, two astronomers* Goujon and Petersen, suspected
its identity with a comet observed in 1532,* and were thus led
to the calculation of an elliptic orbit ; this orbit was actually de-
termined by Goujon, by Briinnow, and later by Bruhns. The
return was predicted for 1851, and the perihelion passage for
November 10 of that year. But the comet was not seen till
* And also with the comet of 1661. But Brorsen's comet is now regarded
as distinct from both these bodies, whose identity is Fuspected, but whose period
is much longer,
119
THE WORLD OF COMETS.
six years later, on its return in 1857, when it made its appear-
ance about three months before its time, for instead of passing
its perihelion on June 25, as required by the ephemeris of Dr.
Galen, it was observed on March 18 by Bruhns, and eight
days later by M. Yvon Villarceau, at Paris, and it was only
after a new calculation of its parabolic elements had been made
that M M . Villarceau and Pape recognised the comet of Brorsen.
The perihelion passage had taken place on March 29, three
months before the predicted epoch, as just stated. This error is
nothing remarkable in the sum of two entire revolutions of a
o
body observed once only. But it explains why the comet was
not seen in 1851, as the place in the heavens to which search
was directed and the time of the search were widely different
from the place which the comet really occupied and the time
at which it passed the perihelion. Instead of November 10 the
date ought to have been fixed for September 27, 1851.
Brorsen's comet, which
was to have reappeared in
1862, 1868, and 1873, was
seen only at its two last
returns. In 1868 the re-
turn calculated by M.
Bruhns, taking into ac-
count the perturbations due
to Jupiter, for April 18,
about midnight, took place
instead on the 17th, about
ten o'clock in the evening.
o
Theory had reasserted its right. The comet was observed at
Marseilles by Mr. Stephan, and at Twickenham by Mr. Bishop,
in the course of September and October 1873.
At its perihelion, Brorsen's comet approaches to within a
distance a little greater than half the distance of the earth from
120
18. — Brorsen's comet, as observed May 14,
1868, from a drawing of Bruhus.
BRORSEN'S COMET.
the sun, viz. to within 55£ millions of miles; at its aphelion it
is beyond the orbit of Jupiter; and its distance from the sun
is then more than nine times as great as its perihelion distance,
viz. about 516 millions of miles. Less eccentric than the
orbits of the comets of Encke, Tuttle, and Halley, the orbit
of Brorsen's comet is more eccentric than that of any other
of the known periodical comets.
121
SECTION VII.
D'AKKEST'S COMET.
Discovery of the comet and of its periodicity by D'Arrest— Return predicted by M.
Yvon Villarceau for 1857 ; verification to -within half a day — Importance of the
perturbations caused by Jupiter— Research of MM. Yvon Villarceau and Leveau
— Return of the comet in September 1870.
HERE, again, we have a periodical comet whose periodicity has
be^n determined by calculation, and whose returns have been
predicted and observed without the help of any comparison
with previous comets. It bears the name of the astronomer
who discovered it in 1851, and who recognised the periodicity
of its orbit. M. Yvon Villarceau had drawn the same conclu-
sion, and calculated the ephemeris for its next return to peri-
helion, which he announced for the end of 1857, a prediction
verified to within twelve hours. The new comet was seen
again at the Cape of Good Hope by Sir Thomas Maclear. On
its following return, which took place, in 1864, astronomers
were less fortunate, and were unable to perceive the comet,
whose position in the heavens and distance from the earth were
very unfavourable. In 1870 the perihelion passage of the
comet took place on September 23 ; it was observed three weeks
before by M. Winnecke, thanks to the ephemeris calculated by
MM. Yvon Villarceau and Leveau.
4 Of all the comets which have not failed to return to us,'
says M. Yvon Villarceau, ' the comet of D'Arrest is perhaps the
122
D' ARREST'S COMET.
most interesting in regard to its perturbations. T do not think
that any other comet has been so closely followed by Jupiter.'
These perturbations, which the above-named astronomer had
calculated for 1864, had increased by more than two months
the duration of the first revolution, the comet being situated
in 1862 at a distance from Jupiter equal to three-tenths of
the distance of the sun from the earth, or at a distance of about
twenty- seven millions of miles. They were calculated with
great care by M. Leveau for the ensuing period, and it is doubt-
less owing to this great work, the labour of three years, that
observations of the comet were rendered possible at its
apparition in 1870. The comet, in fact, passed its perihelion
on September 23 of that year. We enter into these details to
show the difficulties of cometary astronomy, and how science
is able, if not always to surmount them, at least to diminish
them considerably.*
D'Arrest's comet describes its orbit in a little less than six
years and a half (6*567 years), or in 2,398 days, only thirteen
days more than the planet Sylvia. Next to Faye's comet, its
orbit has the smallest eccentricity, or, in other w^ords, the least
elongated figure.
* [M. Leveau lias since performed the calculations for the next revolution of
the comet, and has given an ephemeris for every twentieth day throughout the
year 1877. The perihelion passage is found to occur 1877, May 10'339 Paris
mean time, and the comet will attain its maximum intensity of light about a
fortnight later. It will be nearest to the earth in the middle of October, when
its distance from us will be about one hundred and thirty millions of miles. It
will probably be a very faint object. — ED.]
123
SECTION VIII.
TUTTLE'S COMET.
The period of Tuttle's comet is intermediate to that of Halley's comet and those of
other periodical comets that have returned— Very elongated orbit of the comet
of l.'if years period — Previous observation in 1790; five passages not since ob-
served— Next return in September 1885.
THE periodical comets of which we have just given an account,
and that of Winnecke, which we shall next describe, may be
considered, that of Halley excepted, as comets of short periods;
Tuttle's comet, discovered sixteen years ago by the American
astronomer of that name, is intermediate to Halley's comet of
long period and the others. It performs its revolution in 13|
years, or more exactly in 13 years 29G days, or about 5,044
days — a period nearly two years longer than that which Jupiter
occupies in his revolution. But it describes a very elongated
orbit, so that at its aphelion it is removed from the sun a dis-
tance exceeding ten times its distance at its perihelion; it pene-
trates depths of space that are even beyond the orbit of Saturn ;
in fact, it attains the distance of nearly 955 millions of miles ;
at the perihelion it is about as far distant from the sun as is
the earth.
Tuttle's comet was first observed in 1790 by Mechain, who
discovered it, and by Messier, and it was the comparison of the
parabolic elements of the comets that caused their identity to
be recognised. From 1790 to 1858 there is an interval of sixty-
124
TUTTLE'S COMKT.
eight years ; that is to say, five times the duration of the periodic
revolution of the comet, which must, therefore, have returned to
its perihelion, without having been seen, in 1803, 1817, 1830,
and 1844.
To the calculation of the elliptic elements, performed by
M. Bruhns, we owe our knowledge of the exact period and
the prediction of the return of the comet in the year 1871. It
was, in fact, observed at Marseilles on October 13 of that year,
and afterwards at Carlsruhe, at Paris, and at the Cape of Good
Hope. It passed its perihelion on November 30. Leaving
out of consideration the perturbations the comet may have to
experience in the course of its present revolution, the next
return of Tuttle's comet may be expected in the middle of Sep-
tember 1885. But, as with all other periodical comets, the date
may be somewhat modified * under the influence of the planetary
attractions, and the consequent disturbance of the orbit.
* Tuttle's comet, it should be observed, moves in an orbit the inclination
of which is considerable — it exceeds 54° ; consequently, in withdrawing from
the sun and penetrating to the distances of the larger planets, Jupiter and
Saturn, the comet moves further and further from the paths which they pursue.
The disturbing influence of the masses of these planets upon the comet would,
therefore, in any case, be inconsiderable.
125
SECTION IX.
WINNECKE'S PERIODICAL COMET.
Discovery of the periodicity of the third comet of 1810; calculation of its elliptic,
elements by Encke— Discovery of Winnecke's comet in 1858 ; its identity with
the comet "discovered by Pons- Return of the star to its perihelion in 1869 ;
probable date of its next return in 1876.
IN 1819 Pons discovered, at Marseilles, a comet the elliptic
elements of which were afterwards calculated by Encke; these
elements assigned to it a period of 5T% years. Now, in March
1858 M. Winnecke discovered, at the Observatory at Bonn, a
new comet, whose parabolic elements, it was soon ascertained,
bore considerable resemblance to those of the comet discovered
by Pons. To determine if this identity were real, it was neces-
sary to wait for the comet's return in 1863 and 1869. It was
actually seen in the month of April of the latter year by M.
Winnecke himself, and passed its perihelion on June 30. The
date of its next return is, therefore, approximately fixed for
the month of April 1875.* It will be requisite, however, as
with all comets liable to approach Jupiter or the other planets,
to allow for the perturbations it may have to undergo.
From its first apparition, in 1819, to its last, in June 1869,
is an interval of fifty years, corresponding to nine revolutions
of the comet. Three passages only, as we have seen, have been
* [It was detected by M. Borrelly, at Marseilles, on the morning of February
2, 1875.— En.]
126
WINNECKE'S PERIODICAL COMET.
observed; seven have taken place unperceived. But the elliptic
orbit is now determined with precision. Observers are numerous
and vigilant, and the comet will doubtless no longer escape the
researches of astronomers, except when its apparent proximity
to the sun and its distance from the earth are such as not to
admit of its being seen.
The period of the revolution of Winnecke's comet is 2,042
days, only forty days more than that of Brorsen's comet; the
eccentricity of its orbit is somewhat less. In perihelion the
comet is situated at a distance from the sun equal to four-
fifths of the distance of the earth; in aphelion it exceeds the
orbit of Jupiter by about one-fifth of this distance.
127
SECTION X.
TEMFEL'S SHORT PERIOD COMET.
Calculation of the elliptic elements of the second comet of 18G7, discovered by
Tempel Perturbations due to Jupiter, and consequent delay in the return of the
comet to its perihelion in 1873— Remarkable agreement of observation and cal-
culation.
THE second comet of 1867, discovered by M. Tempel, was
found by several astronomers to have elliptic elements. It
passed its perihelion on May 23, 1867, and its period had been
calculated at 2,064 days. But Dr. Sollinger, taking into ac-
count the perturbations its passage in the vicinity of Jupiter
would produce in the elements of its orbit, assigned a retar-
dation of 117 days in the date of its return to perihelion in
1873. It was, in fact, seen again in the course of that year, and
its perihelion passage took place on May 9, which gives for
the duration of the revolution performed in the interval be-
tween the two apparitions a value of very nearly six years, or
2,178 days, three days less than the number determined by
calculation.
Tempel' s comet of short period is, therefore, the ninth
periodical comet whose return has been verified by observation;
that is to say, which really forms an integral part of our solar
system. Observed in May 1873, at Greenwich, by Messrs.
Christie and Carpenter, it appeared in the telescope like an
elongated nebulosity, about 40" in diameter, with a central
nucleus, which shone like a star of the twelfth or thirteenth
magnitude.
128
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CHAPTER V.
PEEIODICAL COMETS,
129
SECTION I.
COMETS WHOSE RETURN HAS NOT BEEN VERIFIED BY
OBSERVATION.
Periodical comets which have not been seen again; long periods; circumstances
unfavourable to observation ; motions possibly disturbed by perturbations — Elliptic
orbits determined by calculation — Uncertainty of return under these different
hypotheses.
THE nine comets of which we have just given an account are
up to the present time the only comets which can be considered
as certainly belonging to our system. But they are not the
only comets which regularly perform their revolutions round the
sun. Of the numerous comets moving in apparently elliptic
orbits some, we shall now see, have been regarded as new
apparitions of comets previously observed, the great resem-
blance of their parabolic elements having caused their perio-
dicity to be suspected. But either their return to perihelion
has not yet taken place, or circumstances favourable to obser-
vation have not occurred ; or, an equally likely hypothesis,
they may have been disturbed in their courses by the vicinity
of the planetary masses, producing perturbations powerful
enough to change their periods, or even to cast them out of
the sphere of the sun's attraction, of which perhaps until then
they had formed for a time a part.
Other comets, which have not been assimilated to comets
already observed, have elliptic orbits determined by calculation ;
131 K 2
THE WORLD OF COMETS.
but for the reasons that we have just enumerated they have
not been seen again ; that is to say, they have periods much too
long, or they have been subjected to disturbing causes.
We will now pass in review the principal comets of these
two categories, and for greater clearness we will divide them
into three classes : —
1. Comets of short period] that is to say, those which per-
form their revolution in a few years, like the eight periodical
comets above described. All cornets of this first class are interior
comets, because their orbits do not exceed the known bounda-
ries of the planetary orbits ; in other words, because at their
aphelia they are at a less distance from the sun than Neptune.
2. Comets of mean period ; that is to say, those which de-
scribe their orbits in less than two centuries, like Halley's
comet. Comets of this class are not interior comets.
3. Comets of long period, whose revolution exceeds two
centuries, and may amount to hundreds of thousands and even
to millions of years. Comets of this class penetrate to remote
depths of space far exceeding the limits of the solar system.
132
SECTION II.
INTERIOR COMETS, OR COMETS OF SHORT PERIOD, THAT HAVE
NOT YET RETURNED.
Comets lost or strayed : the comet of 1743 ; the comet of Lexell, or 1770; perturbations
caused by Jupiter ; in 1767 the action of Jupiter shortens the period, and in 1779
produces an opposite effect — Comet of De Vico; short period comets of 1783,
1846, and 1873.
DURING the month of February 1743 a comet was observed at
Paris, Bologna, Vienna, and Berlin whose parabolic elements
were calculated by Struyct and Lacaille. A mathematician
of our time, M. Clausen, identified it as a comet of short period,
performing its revolution in five years and five months. Is
this, as has been supposed, the same comet that was seen in
November 1819? If so, its period must have greatly changed,
since the calculations of Encke assign to the latter a period of
about four years and ten months.
The comet of which we are now about to speak is cele-
brated in the history of astronomy. The following extract
from a memoir by M. Le Verrier gives an account of the cir-
cumstances of its first apparition : —
' Messier perceived, during the night of the 14-15th of
June, 1770, a nebulosity situated amongst the stars of Sagit-
tarius, but not discernible by the naked eye ; it was a comet
first coming into view. On the 17th of June it appeared sur-
rounded by an atmosphere the diameter of which was about
133
THE WORLD OF COMETS.
5' 23". In the centre appeared a nucleus; its light was
bright, like that of the stars. Messier estimated its diameter
at 22 seconds.
1 The comet rapidly approached the earth. On the 21st
of June it was visible to the naked eye, and three days
after it shone like a star of the second magnitude. The
diameter of the nebulosity, which was not more than 27',
gradually increased, and by the night of the lst-2nd of July
had attained 2° 23'. But, whilst the diameter of the nebulosity
thus increased, according to the laws of optics, in the inverse
ratio of the distance of the body from the earth, the diameter
of the supposed nucleus remained, on the contrary, nearly
invariable.
1 From the 4th of July the comet became lost in the rays
of the sun and remained for a short time invisible. Pingre,
from the observations of Messier, calculated a parabolic orbit.
The comet, it was found, would again become visible in the
month of August, and Messier was once more able to observe
it on the 4th of that month. From this date it was seen almost
without interruption ; but as it gradually drew further away
from the sun and the earth it faded out of sight, and was lost
to view in the first days of October.
' Before the time of the perihelion passage no indication of
a tail had been perceived. But from the 20th of August to
the 1st of September the comet was provided with a faint tail,
the length of which was about one degree.
' The parabolic elements given by Pingre were in accord-
ance with the first observations, but they differed greatly from
the last. Nor were other elements calculated by Slop, Lam-
bert, Prosperin, and Widder more exact. The discrepancies
were generally referred to the derangement of the orbit caused
in June by the action of the earth. Prosperin, however, sus-
pected that the orbit of the comet might be elliptic, but he
134
• INTERIOR COMETS.
contented himself with the hypothesis and did not verify it.
Lexell at last discovered that the comet was moving in an
elliptic orbit, which it described in 5*585 years (a little more
than five years and a half) : and rejecting, with Dionis du
Sejour, the supposition that the disturbing action of the
earth could have considerably changed this orbit, he proved,
first, that the observations were all satisfied by a period of five
years and a half; second, that it was impossible to admit a
period of five or six years without introducing differences into
the theory incompatible with the observations.
4 " But," said Messier, " if the duration of the comet's revo-
lution is only five years and a half, how is it that it has only
once been observed? This is a very strong objection to the
researches of M. Lexell."
'Lexell replied, "As the aphelion distance of the comet
from the sun is nearly equal to the distance of Jupiter from
the sun, a suspicion arises that tbe comet may have been dis-
turbed in its movement by the action of Jupiter, and that at
one time it described an orbit altogether different from that in
which it moves at present. It is found by calculation that this
comet was in conjunction with Jupiter on the 27th of May,
1767, and that the distance between the comet and the planet
was only ^^th of the distance of the comet from the sun ;
hence we may conclude, bearing in mind the masses of Jupiter
and the sun, that the action of Jupiter has been powerful enough
to change the movement of the comet in a sensible degree."
Lexell further pointed out that the comet would be in proximity
to Jupiter about the 23rd of August, 1779, and that this cir-
cumstance might perhaps prevent the comet returning to its
perihelion in 1781, as it would do if undisturbed by pertur-
bations. And, in fact, the astronomical world vainly awaited
the return of the comet in 1781 and 1782.'
Since the end of the last century Lexell's comet, or the
135
THE WORLD OF COMETS.
comet of 1770, has not been seen. It is, therefore, a comet lost
or gone astray, and it is easy to conceive the fascination of the
problem offered to astronomers by the fact of its disappearance.
Several mathematicians following Lexell have attempted its
solution Burckhardt, Laplace, and M. Le Verrier himself.
According to Laplace the action of Jupiter in 1767 rendered
the comet visible by diminishing its perihelion distance, and
the same action in 1779, by increasing the same distance, has
rendered it invisible for ever.
M. Le Verrier has discussed the question anew; he sub-
jected to enquiry the amount of the perturbations the earth
would have caused in 1770, when the comet approached it to
within a distance not exceeding seven times that of the moon.
Proceeding next to estimate the disturbing influence of the
mass of Jupiter upon the comet at its aphelion passage in
1779 and during the next two years, he has shown that the
perturbations produced during the twenty-eight last months
were considerable and must have completely changed the
orbit of the comet.
4 From the 28th of May,' he proceeds, ' the comet was
rapidly approaching Jupiter in a hyperbolic orbit, and it is
impossible that the comet should have become a satellite of
Jupiter, as has been supposed.' Did it strike against that
powerful mass, or, at all events, did it traverse the regions
where the four satellites describe their orbits? To these
questions M. Le Verrier replies that 'it is not absolutely im-
possible, but that it is very improbable, and that conclusions
in the affirmative, based upon the diminutive size of the
comet's mass, are very hazardous.'
According to his first memoir there is every reason to
believe that the comet of 1770 has not been carried away from
our solar system.
But if so, would it not have been seen again? Among the
J36
INTERIOR COMETS.
number of comets which have been seen since 1780 are there
none identical with Lexell's comet? It is not surprising that
no comet has appeared with elements similar to those of Lexell's
comet, if we bear in mind the perturbations which it under-
went during its return to aphelion, in 1779. A first examina-
tion seemed to indicate a similarity between the comets of
Lexell and Faye. But M. Le Verrier has demonstrated the
contrary, by tracing back the course of the latter comet before
its discovery in 1843, and has shown that ; we must go back
to the year 1747 for the time when the comet of Faye began
to describe (under the influence of Jupiter) the contracted
ellipse in which it moves at the present day.'
Here, then, is a comet which is as lost to our world, or at
least to astronomers, for even if it should return how would
it be possible to recognise its identity ?
A comet was discovered on August 22, 1844, at Rome, by
the astronomer De Vico. The elliptic elements calculated by
Faye and Brunnow proved that its period of revolution
was five years and a half only (or more exactly, 1,996 days) ;
so that, but for perturbations, the comet would return to its
perihelion in February 1850, again in August 1855, in
January 1861, July 1866, December 1871, and to its next
perihelion in June 1877. It has not been seen, ho \\ ever,
either on the first of its probable returns or subsequently.
It is, therefore, at all events, a comet that has gone astray.
It has also been supposed that the comet of De Vico is a
new apparition of that of Lexell, with which it has points of
vao-ue resemblance. Acccording to M. Le Verrier the two
O <->
comets are entirely distinct. Nor does he admit the conclu-
sions of Mauvais and Laugier, who consider the comet of
1844 identical with that of 1585. But he considers it very
probable that the comet observed in 1678 was De Vico's
comet. The following are his conclusions on this point, written
137 '
THE WORLD OF COMETS.
in 1847, before the time fixed for the first return of De Vice's
comet : —
' The comet of 1844 might have come to us, as others have
come, from the furthest regions of space, and have been
attached to our system by the powerful influence of Jupiter.
The date of its arrival may doubtless be referred back many
centuries. Since that epoch it has often passed in the neigh-
bourhood of the earth, but in all this time it has only once been
observed — 166 years before the apparition of 1844 (viz. at the
apparition of 1678, mentioned above). This comet will for a
length of time move in the restricted orbit it now describes.
o
In a certain number of ages, however, it will again reach the
orbit of Jupiter, in a direction opposite to that in which it
may have first entered the solar system, and its course will
once more be altered. Perhaps Jupiter himself will restore it
to the regions of space from which he had previously appro-
priated it.'
At the epoch of its greatest visibility, which took place in
September, the comet of De Vico was for several days per-
ceptible to the naked eye. In the telescope it presented a
remarkable peculiarity: the nebulosity, which was fan-shaped,
contained a circular nucleus, pretty well defined ; it had a tail —
of bluish tint, but of no great length — which pointed from the
sun.
Amongst the comets of short period whose orbits have
been calculated, but which have not returned, we have yet to
mention the comet of 1766, which, according to Burckhardt,
performs its revolution in five years, and is perhaps a previous
apparition of the comet discovered by Pons in June 1819.
This latter, according to Encke, would have a period of 5*6
years, the orbit having in the interval been changed by the
planetary perturbations. Next comes the comet discovered
by Peters on the 26th of June, 1846. Its time of revolution
138
INTERIOR COMETS.
is about sixteen years, but it was not seen in 1862, and
will have to be again looked for in 1878. Lastly, a comet
was seen at Marseilles in 1873 by M. Stephan, moving in an
elliptic orbit, and with a period of 1,850 days, or a little more
than live years. In the course of 1878 this comet may, there-
fore, be expected ; and should it reappear it will be the tenth
periodical comet belonging to the solar system, whose return
has been observed, or even the eleventh, if the comet discovered
by Peters should also return.
Four other periodical comets (one of which we shall refer
to again when we explain the connexion existing between
comets and shooting stars) must be placed amongst the number
of interior comets — we can hardly say of short period comets
— not yet seen again. One is Tempel's comet I., 1866, which
performs its revolution in a period of 33*176 years, or thirty-
three years and sixty-four days. It passed its perihelion on
January 11, 1866, and will consequently return in the spring
of 1899. This comet approaches the sun to within a distance
rather less than that of the earth; but at its aphelion it is far
beyond the orbit of Uranus. The comets of 868 and 1366 are
very probably anterior apparitions of this thirty-three years
comet. Since the first of these dates it has, therefore, re-
turned to its perihelion twenty-nine times without having
been perceived, and has thus effected at least thirty entire
revolutions round the sun. A second comet — the first of the
year 1867 — has also a period of more than thirty- three years
(33-62 years, or rather more than thirty-three years and a
half). At its aphelion distance, which is equal to nineteen and
one-third times the distance of the sun from the earth, it is far
beyond the orbit of Uranus, but at its descending node (which
it passes through about 5,800 days after perihelion passage) the
two orbits are situated very near together, the distance being
scarcely 2,237,000 miles. In 1817, but more especially in
. 139 /
THE WOULD OF COMETS.
* 1649, the comet in passing through its node was situated in
the close vicinity of Uranus, and hence must have been
produced very considerable perturbations in the movement of
the former. It should be looked for again in the year 1900.
Lastly, there are two comets with nearly the same period of
fifty-five years. The one was discovered in 1846 by De Vico,
and should effect its first return in the year 1902; at its aphe-
lion it withdraws to a distance nearly equal to the radius of
the orbit of Neptune; it is thus an interior comet. The other
comet of fifty-five years, which likewise does not pass beyond
the orbit of Neptune, was discovered in 1873, by M. Coggia,
at Marseilles ; it is suspected to be identical with a comet
observed by Pons in 1818.
140
SECTION III.
COMETS OF MEAN PERIOD.
Periodical comets exterior to the solar system ; the type of this class is Halley's comet,
which is the only comet of mean period whose return has been verified by
observation — Enumeration of comets with periods between 69 and 200 years —
Periods ; aphelion and perihelion distances.
OF the comets belonging to this class Halley's comet is the
type; but it is the only one of which we have several un-
disputed apparitions. When a comet is suspected to be
identical with some other comet that has been previously
observed, from the similarity of the parabolic elements, its
return is probable; but as a rule great uncertainty attaches
to the length of the period, even if, assuming the identity of
the two comets, the perturbations be left out of the question.
A third apparition is, therefore, generally necessary before the
identity and real periodicity of a comet can be affirmed. And
this third element up to the present time is wanting in the
comets we are now engaged upon. But it will evidently suffice
to prove a second apparition, when the elliptic elements have
been calculated solely from observations of the first apparition.
The following, in the order of their discovery, are the
nine comets of mean period which we have to mention : —
The first on the list is the comet of 1532, observed by
Apian and by Fracastoro, ' whose head,' says the latter ob-
server, ' was three times larger than Jupiter, with a beard two
141
THE WORLD OF COMETS.
fathoms long.' According to the calculation of Olbers this
comet has a period of 129 years, and is identical with a comet
which appeared in 1661, and at several other remarkable
epochs. Sir John Herschel mentions it in the following
manner: 'In 1661, 1532, 1402, 1145, 891, and 243 great
comets appeared, that of 1402 being bright enough to be seen
at noonday. A period of 129 years would reconcile all these
appearances, arid should have brought back the comet in 1789
or 1790. That no such comet was observed about' that time
is no proof that it did not return, since, owing to the situation
of its orbit, had the perihelion passage taken place in July it
might have escaped observation.' Its next return should
take place between 1918 and 1920.
A comet observed by the English astronomer Flamsteed,
from the end of July to the commencement of September
1683, has, according to the elliptic elements calculated by
Clausen, a period of about 190 years. This comet, whose return
ought to have been observed about 1870, has failed to
reappear; but the perturbation caused by the larger planets
might occasion a delay of several years, and its re-appearance
may still be expected. At its aphelion it recedes far beyond
the orbit of Neptune, surpassing it by 497 millions of miles.
A new discussion of the observations, however, by Mr. Plum-
mer affords a presumption that the comet of 1683 describes a
parabolic orbit, in which case it should be removed from the
list of periodical comets.
About the years 1882 and 1887 search will have to be
made for two comets, the first of which, discovered by Pons, in
July 1812, has a period of about seventy-one years, and the
second, discovered by Olbers, in March 1815, has a period of
about seventy-four years, as calculated by Bessel. The return
of the comet of 1815 to its perihelion would be accelerated
two years by the perturbations of the planets. Then comes
142
COMETS OF MEAN PERIOD.
the comet discovered in February 1846 by De Vico and
Bond; a period of seventy -three years would bring it back to
perihelion about the middle of the year 1919. Next we have
the comet discovered by Brorsen (July 1847), with a period
of seventy-five years, to return in 1922; that of Westphal
(July 1852), with a period of sixty-one years, and next return
in 1913; that of Secchi (comet I., 1853), whose period would
be 188 years, and which, according to Mr. Hind, has a great
resemblance to the comet of 1664. Lastly, the third comet of
1862, of whose connexion with the meteor stream of August
o
10 we shall have to speak in a later chapter. This comet has
a period of about 120 years; its next appearance, therefore,
should be expected about 1982.
We now append in order, according to the duration of
their periodical revolutions, a list of the nine comets above
enumerated. They would be ten in number, if Halley's comet,
which we have taken as a type of this class (and be it noted
that this division into classes is quite arbitrary), had not
been placed amongst comets of verified return. We also
add to this enumeration their greatest and least distances
from the sun, expressed both in mean radii of the earth's
orbit and in miles : —
Comet.
Period.
Perihelion distance.
Aphelion distance.
(radii.) (miles.)
(radii.) (miles.)
1852 II.
61 years
1-25
115,800,000
29-61
2,713,200,000
1812
71 „
0-78
70,800,000
33-40
3,060,900,000
1846 III.
73 „
0-66
60,500,000
34-40
3,152,900,000
1815
74 „
1-21
110,600,000
34-10
3,125,600,000
1847 V.
75 „
0-49
46,600,000
35-10
3,217,500,000
1862 III.
120 „
1-01
92,600,000
48-70
4,463,300,000
1532
129 „
0-61
55,900,000
48-05
4,403,800,000
1853 I.
188 „
1-03
94,400,000
65-02
5,959,100,000
1683
190 „
0-55
50,400,000
65-50
6,003,900,000
143
SECTION IV.
COMETS OF LONG PERIOD.
Periodical comets exterior to the known limits of the solar system — Distance to which
the comet of longest calculated period recedes from the sun— The so-called comet
of Charles V. : its apparitions in 1264 and 1566 ; its return predicted for the middle
of the nineteenth century, between 1848 and 1860 — Calculation of the perturbations ;
another comet lost or strayed— The great comet of 1680: the Deluge and the end of
the world— Magnificent comets of 1811, 1825, and 1843.
OF the comets we are now about to mention, the periods
of which have been calculated approximately, none cer-
tainly will be seen by anyone now living. One alone was ex-
pected about fifteen years ago ; and if it really did return to
its perihelion, in spite of all researches it was not observed,
and it will not be visible again until after the lapse of three
centuries.
We will begin by enumerating the comets, and will after-
wards give some details about the most remarkable of them.
The following tables contain the durations of their revolutions
and their distances from the sun, expressed in radii of the
terrestrial orbit : —
144
COMETS OF LONG PERIOD.
Probable
previous
appari-
tions
Period of
revolutions
Distan
Perihelion
•es from the Sun
Aphelion
Comet of 1845 III.
1596
249 years
0-401
78-38
„ 1556
1264
292 „
0-500
87-53
„ 1840 IV.
—
344 „
1-481
96-76
„ 1843 I.
—
376 „
0-006
104-28
„ 1846 VI.
—
401 „
0-633
108-21
„ 1861 I.
—
415 „
0-921
110-40
„ 1861 II.
—
422 „
0-822
111-70
„ 1793 II.
—
422 „
1-495
111-03
„ 1746
1231
515 „
0-950
127-55
„ 1840 III.
1097
743 „
0-742
163-20
„ 1811 II.
—
875 „
1-582
181-44
„ 1860 III.
—
1,000 „
0-292
211-30
„ 1807
—
1,714 „
0-646
286-07
„ 1858 III.
—
1,950 „
0-578
311-40
„ 1769
—
2,090 „
0-123
326-80
., 1827 III.
—
2,611 „
0-138
379-10
„ 1846 I.
—
2,721 „
1-481
388-32
„ 1811 I.
—
3,065 „
1-035
421-02
„ 1763
—
3,196 „
0-498
434-32
„ 1825 III.
—
4,386 „
1-241
534-64
„ 1864 II.
—
4,738 „
0-909
563-30
„ 1822 III.
—
5,649 „
1-145
618-15
„ 1849 III.
—
8,375 „
0-895
812-73
1680
—
8,813 „
0-006
855-28 ,
„ 1840 II.
—
13,866 „
1-221
1,053-00
1847 IV.
—
43,954 „
1-767
2,489-03
„ 1780 I.
—
75,838 „
0-096
3,974-88
„ 1844 II.
—
102,050 „
0-855
4,366-74
„ 1863 I.
—
1,840,000 „
0-795.
29,989-00
„ 1864 II.
1
2,800,000 „
0-931
40,485-00
We scarcely need warn the reader that the periods of
the comets in this division are far from being well determined.
In some 'cases the periodicity has been determined from the
similarity presented by the elements of the orbits to those of
preceding comets, and in others by a direct calculation of the
elliptic elements. But even when the calculation rests upon
a very sure basis, as is the case with several, it must be
remembered that the next returns deduced from the periods
given are subject to modification from the casualties of the
145 L
THE WORLD OF COMETS.
voyage ; that is to say, from perturbations which may be expe-
rienced on the journey from known and unknown planets of
the solar system.
The second comet of the foregoing table is interesting, not
only from an astronomical but also from an historical point of
view. The following are some details concerning the history
of its apparitions.
About the middle of July, after sunset, there appeared
in France, in the year 1264, a comet, which Pingre', in his
Cometographie, calls a l great
and celebrated comet.' Se-
veral causes contributed to
its celebrity At the epoch
of its first apparition super-
stitious beliefs in cometary
influences were still rife, and,
as we may well believe, these
were not diminished by this
apparition, for after exhibit-
ing itself in Europe for two
months and a half it disap-
peared on October 3, 'the
very day on which Pope
Urban IY. died.' Eye-wit-
nesses who attest this fact did
not fail to conclude ' that it
Fig. 19. Great comet of 1264, from Theatrum na(l only appeared to an-
Cometicum of Lubienietzki. ,1 • j ,1 , T ,-,
nounce this death. In the
last century .Dunthorne and Pingre calculated the parabolic
elements of the comet's orbit, which they found bore great
resemblance to those of the comet of 1556. 'The comet of
1264,' says Pingre, 'is very probably the same as that of 1556;
146
COMETS OF LONG PERIOD.
its periodical revolution is about 292 years, and its return
may consequently be expected about 1848.'
This identity, which till quite recently was considered beyond
dispute, would make the comet in question a most formidable
body, since, after presiding at the death of a Pope, it came to
decide the abdication of a famous sovereign. It is known in
history as the comet of Charles V., and is thus mentioned in
the Cometographie : —
1 The apparition of this comet produced, according to several
writers, a very singular effect. It struck terror into the Em-
peror Charles V. ; this prince doubted not his death was at
hand, and is said to have exclaimed —
His ergo indiciis me mea fata vocant.
This verse has been translated into French :• —
Par la triste comete,
Qui brille sur ma tete,
Je connois que les cieux
M'appellent de ces lieux.
For this translation, which is open to improvement, Pingre
has proposed to substitute the following : —
Dans ce signe eclatant je lis ma fin prochaine.
Be this as it may, ' if the historians I have quoted,' Pingre
continues, ' are to be believed, the panic contributed not a little
to the design which Charles V. formed, and executed a few
months later, of ceding the imperial crown to his brother Fer-
dinand ; he had already renounced the crown of Spain in
favour of his son Philip. If this account be true, the fact
deserves to be added to the numbers of great events produced
by very little causes.'
But is it true? The tradition, it is certain, was current a
few years ago, as the following passage will testify, taken from
a lecture given by M. Babinet at one of the public seances of
147 i 2
THE WORLD OF COMETS.
the Institute : ' In 1556 a great and beautiful comet appeared.
Charles V., who had hitherto delayed his abdication, hesitated
no longer. To him, as the greatest of living sovereigns, the
comet was addressed. The influence which menaced the em-
peror would, he hoped, be divested of evil for the private
individual— would fall harmless upon the monk.' Was this,
we may ask, the decisive reason that determined the famous
emperor to retire to the cloisters of St. Justus ? Such is not
the opinion of M. Mignet, who has established the fact that
Charles Y. abdicated in 1555, and that consequently ' it was
not the fear of the hairy star of 1556 which caused him to
descend from the throne.'
Leaving history, however, let us return to science, and to
the scientific reasons which have drawn the attention of savants
to the comet of 1264 and 1556. It has just been seen that in
the eighteenth century its return was predicted for the year
1848. Encke believed its return was possible in 1844, and
that comet III. of 1844 was identical with that of Charles V.
Mr. Hind, on the contrary, having calculated the elliptic ele-
ments of comet III., assigned to it a period of 41 1 years. But
this period may not be incompatible with the three dates of 1264,
1556, and 1844 ; for the first interval, 1264-1556, would give
seven periods of 41J years ; and the second, 1556-1844, would
give, pretty nearly, the same number of revolutions. An
acceleration of four years, in so long a time, distributed
moreover amongst several successive revolutions, is not at all
inadmissible. Sir John Herschel, in the sixth edition of his
Outlines of Astronomy, published in 1868, expresses himself
in these terms on the subject of the supposed identity of the
comets of 1264 and 1556 : ' Mr. Hind,' he says, ' has entered
into many elaborate calculations, the result of which is strongly
in favour of the supposed identity. This probability is further
increased by the fact of a comet, with a tail of 40° and a head
148
COMETS OF LONG PERIOD.
bright enough to be visible after sunrise, having appeared in
975 ; and two others having been recorded by the Chinese
in 395 and 104. It is true that, if these be the same, the
mean period would be somewhat short of 292 years. But the
effect of planetary perturbation might reconcile even greater
differences ; and though even to the time of our writing (1858)
no such comet has yet been observed, two or three years must
yet elapse, in the opinion of those best competent to judge,
before its return must be considered hopeless.'
Let us finish the history of this celebrated comet and the
efforts that have been made to re-discover it. Mr. Hind began
by calculating the amount of perturbation the comet would be
subjected to in 1556 by its passage in the vicinity of the earth.
Its return was first expected in 1848. ' But 1849, 1850, 1851,
and 1852 have passed, and the great comet has failed to appear !
Here, however, is news of it at last ' — these lines were written
by M. Babinet, in March 1853 — ' which I take from Mr. Hind's
excellent treatise, that I have just received. It is due to M.
Bomme, a learned mathematician of Middleburg (Zealand),
who appears to have resolved the question completely. Dis-
satisfied, like all astronomers, at the non-arrival of the comet, M.
Bomme has performed de novo the whole of the calculations,
and estimated the separate action of each of the planets upon
the comet for 300 years of its revolution, month by month
and day by day, when necessary. M. Bomme, aided by the
preparatory work of Mr. Hind, with a patience characteristic
of his countrymen, has re-calculated, at a great expenditure of
time and labour, the entire path of the comet.'
The result gave for the epoch of its return to perihelion the
month of August 1858, with an uncertainty of two years either
way. But in vain was it looked for ; astronomers swept with
their telescopes every region of the heavens. Splendid comets
appeared in 1858, 1861, and 1862, but the comet of Charles V.
149
THE WORLD OF COMETS.
never returned. Mr. Hind and M. Bomme had not the same
good fortune as Clairaut, Lalande, and Mdlle. Lepaute, in the
last century. Like Lexell's comet and De Vice's comet of
short period, the comet of 1264 and 1556 must be considered
lost • and if in reality merely accidental causes prevented its
being observed, and it should appear again, it will be our
descendants in the twenty-second century who will have the
satisfaction of celebrating its return.
Amongst other comets of long period must be mentioned
the great comet of 1680, made famous by the hypothesis of
Whiston, who assigned to it a revolution of only 575 years,
and thus made one of its previous apparitions coincide with
the date of the Deluge. The Deluge, according to Whiston,
was caused by a rencontre between the earth and this formi-
dable comet, which, being destined to destroy our globe by fire,
after having first drowned it, is to bring about the end of the
world. Further on we shall return to these fancies. Accord-
ing to the calculations of Encke the comet of 1680 has a period
of more than eighty-eight centuries. At its aphelion it would
be distant from the sun and the earth 850 radii of the earth's
orbit, or about 77,673 millions of miles. ' At this enormous
distance,' says Humboldt, ' the comet of 1680, which at its
perihelion has a velocity of 244 miles per second — that
is to say, thirteen times greater than that of the earth —
moves at a rate hardly greater than ten feet per second ; that
is, scarcely more than triple the speed of our European rivers,
and only the half of that which I have myself observed in the
Cassiquiare, a branch of the Orinoco.' It was the comet of
1680 which furnished Newton with the elements of his theory
of cometary movements. Of all known comets it is, after the
one which we mention next, that which approaches most nearly
to the sun; its perihelion distance being 0*0062. The peri-
helion distance of the great comet of 1843 is 0-0055, which
150
COMETS OF LONG PERIOD.
is equivalent to only about 506,000 miles measured from the
centre of the solar sphere. Thus, the nuclei of these two
famous comets have passed respectively, the one to within
143,000 and the other to within 78,000 miles of the surface
of the sun, and have, therefore, certainly passed through that
hydrogenous atmosphere the existence of which the corona in
total eclipses has revealed to us.
The great comet of 1769, which was observed in Europe,
in the island of La Reunion, and at sea, near the Canaries,
has, according to the calculations of Euler, Lexell, and Pingre,
an elliptic orbit, but there is an uncertainty in the period of from
450 to 1,230 years. Bessel, after a profound discussion, fixed its
Fig. 20.— Great comet of 1811.
most probable period at 2,090 years ; but there remains an
uncertainty at least of 500 years. Similar uncertainty exists
with regard to the period of the comet of 1843, which, if
identical, as has been believed, with that of 1668, would have
a period of 175 years instead of 376; and also with regard
151
THE WORLD OF COMETS.
to the comet of 1793, whose period was first calculated at
twelve years. That of 422 years, which we have given, fol-
lowing D' Arrest, is the result of a more careful investigation.
The. period of De Vice's comet (1846)— viz. 2,721 years-
is not more certain ; the approximation is only to within 400
or 500 years either way. The comet of 1840, whose period we
have given as nearly 14,000 years, has, according to Mr.
Loomis, a period of only 2,423 years. We have seen above a
similar kind of difference in regard to the great comet of 1680.
But the most recent discussions of cometary elements of all
kinds should inspire more confidence, and for this reason we
have used them in preference to the older determinations.
Two comets amongst those in the preceding table still
remain to be noticed. The first is the great comet of 1825, or
the comet of Taurus, which was visible for nearly a year — from
July 15, 1825, the day of its discovery by Pons, to July 5,
1826, the last day on which it was seen ; the other is that of
1811, the great comet which was also observed in 1812, and
which is so well remembered in Europe — in the West, on
account of the excellent wine attributed to it, and generally
known as the Comet Wine, and in the East, because it was
regarded by the Russians as a presage of the great and fatal
war of the first Napoleon against Russia.
Comets of long period have nothing to distinguish them
from other comets, except the enormous distances to which
" they recede from the sun at the time of their aphelion. The
smallest of their orbits exceeds the known limits of the solar
system by more than forty-eight times the mean distance of
the sun from the earth. The comet of 1845 recedes to a dis-
tance from the sun of two and a half times the distance of
Neptune ; that is to say, to a distance of 6,260 millions of miles.
The comet of 102,000 years period penetrates to a distance
fifty-five times greater still. Finally, the two last comets of
152
«*•
CO
u
C
O
UJ
O
cc
u
I
J
COMETS OF LONG PERIOD.
our table which perform their revolutions, the one in 18,400 cen-
turies, the other in 28,000 centuries, reach at their aphelia to
regions of space so remote that their light would require
171 days and 230 days respectively to reach our earth.
The comet of 1864 (the last on the list) attains a distance
from the sun equal to one-fifth of the distance of the star
Alpha Centauri from our system, the distance of this star
being assumed to be equal to 200,000 times the mean distance of
the sun from the earth. The voyage outward, it is true, takes
1,400, 000. years, and the return also 1,400,000 years. It has
been said of the comet of 1844 (that of 102,000 years period)
that it has left us for depths of space more remote than Vega,
CapeUa, or Sirius. This is not true ; it would not be true even
for comets with periods measured by millions of years ; but
there is nothing to prevent it being so for comets which
have hyperbolic or parabolic orbits.
[Reference is made to certain points in this chapter and in the next, in the
editor's " note upon the designation of comets and the catalogue of comets,"
which will be found at the end o£ the volume. — ED.]
15.3
CHAPTEE VI.
THE WORLD OF COMETS AND COMETAEY SYSTEMS.
SECTION I.
THE NUMBER OF COMETS.
Kepler's remark upon the number of comets — Comets observed — Comets calculated
and catalogued — Conjecture as to the number of comets which traverse the solar
system or belong to it ; calculations and estimates of Lambert and Arago — Calculation
of the probable number of comets from the actual data ; Kepler's remark verified.
' COMETS are as numerous in the heavens,' said Kepler, l as
fishes in the ocean, ut pisces in oceano? In quoting this com-
parison of the great astronomer we only follow the invariable
custom of all the authors who have hitherto treated the ques-
tion of the number of comets ; but we remark that the expres-
sion employed by Kepler is only the result of an opinion which
is little more than a conjecture, and that the words ought to
be taken in their poetical rather than in their literal sense ; but,
making allowance for some exaggeration in the expression, we
shall see that Kepler was justified in considering the number
of comets as very great.
Our inquiry, it is evident, must be confined to comets
which are liable temporarily to traverse our system, or to revolve
for ever about the sun as an integral part of the solar system.
Any attempted estimate of comets situated outside this sphere,
beyond our range of vision, and exterior to the planets which
belong to our group, could not rest upon any certain data.
Our calculations and conjectures must be limited to the domain
of that which admits of proof, and is strictly within the power
157
THE WORLD OF COMETS.
of observation. Beyond this limit number fails us — we lose
ourselves in the infinite.
Let us, in the first place, speak of comets which have been
observed or at least of those which have been noted by history
and tradition. The following is a passage from Lalande,
acquainting us with their number as it was known in the last '
century: ' Riccioli,' says he, 'in his enumeration of comets,
reckons only 1 54 as mentioned by historians up to the year
1651, in which year he composed his Almagest, the last having
appeared in 1618. But in the great work of Lubienietzki, in
which all the passages to be found in any author having the
slightest reference to a comet are scrupulously recorded, we
find 415 apparitions, up to the comet which appeared from
the 6th to the 20th of April, in the year 1665. Since that
time forty-one have been observed, including those which
appeared in the year 1781.' This makes in all, therefore, up
to this last date, 461 comets.
This number has been much increased since, partly by the
apparition of new comets and partly by the researches of scholars
and the study of the Chinese annals, which have brought to
light many apparitions of comets forgotten or not observed in
Europe. The following is a table based upon that published
by Mr. Hind in 1860, and completed to the present time: —
Comets Comets recognised as
calculated re-apparitions
4 1
1 1
2 1
3 2
0 1
1 1
4 1
0 2
2 1
1 1
2 ' ..-: 3
Comets
observed
Before our era
68
First century
21
Second „
24
Third „ . .
40
Fourth „ .
25
Fifth „ .
18
Sixth „
25
Seventh „ .
31
Eighth „ .
15
Ninth „
35
Tenth „ .
24
158
THE NUMBER OF COMETS.
Comets Comets Comets recognised as
observed calculated re-apparitions
Eleventh century .31 3 2
Twelfth „ . 26 0 l
Thirteenth „ .27 3 3
Fourteenth „ . 31 8 3 '
Fifteenth „ . 35 6 1
Sixteenth „ .31 13 5
Seventeenth „ .25 20 5
Eighteenth „ .69 64 8
Nineteenth „ . 189 189 42
Total . ..; 790 326 85
• Deducting from the total number of 790 comets observed,
those which have returned, some several times, we find altoge-
ther 705 distinct comets. With regard to this number we must
bear in mind that up to the sixteenth century all comets were
observed with the naked eye, and that since the invention of
the telescope a great number have been discovered by its
means. The preceding table gives, therefore, up to about the
year 1600, the most brilliant comets only; but subsequently
telescopic comets, too faint or too far from the earth to be
visible to the naked .eye, have outnumbered the others. In
the sixteenth century thirty-one comets were observed, of which
eight were telescopic. In the seventeenth century the number
of telescopic comets amounted to thirteen out of twenty-five;
to sixty-one out of sixty-nine in the eighteenth century; and
in the three-quarters which have elapsed of the nineteenth
century, out of 189 comets fifteen only have been visible to the
naked eye; 174 have been discovered or recognised by the
help of instruments, thanks to the zeal of numerous astrono-
mers who have occupied themselves with such researches in
both hemispheres.
The progress, however, accomplished by astronomers in
our own and the last century consists not so much in the
number of discoveries as in the determination of orbits, the
159
THE WORLD OF COMETS.
precision of the observations, and the study of the physical
constitution of these bodies. Before the time of Newton we
must distinguish between the number of comets observed or
simply seen and the number catalogued. These last are not
very numerous before the sixteenth century, because the
ancient historians have left but very inexact records of the
positions and movements of the comets of their time ; and
indeed the documents which have since rendered possible the
calculation of the orbits are chiefly those of the Chinese
annalists. On the other hand, for the last three and a half
centuries nearly all comets observed have been catalogued.
In our century this is true of all. Except in the unusual case of
the apparition being so brief that three observations, separated
by the requisite intervals, cannot be obtained, when a comet
is seen and observed its elements are now promptly calcu-
lated.
But to return to the number of comets that have been cata-
logued. It amounts to 326, a number, it is true, which must be
reduced to 241, if we desire to take into consideration distinct
comets only. As regards the probable number of comets
which have made their appearance in the solar system in
historic times, it is clear that it must be considerably greater
than the number given above, even if we take into considera-
tion those comets only which have crossed our system under
favourable conditions of visibility for an observer situated on
the surface of the earth. To explain this let us take as a
basis the number afforded by the present century. It amounts,
in the three-quarters of a century, to 147 comets, which gives,
therefore, 185 comets for the entire century. This number is
well within the truth, and we might even assume without
exaggeration an average of two new comets per year. But,
restricting ourselves to the number 185, and confining our
calculation only to the last twenty centuries, we thus obtain a
](50
THE NUMBER OF COMETS.
total of 3,700 comets — an enormous number, but one which
must be still further augmented, for the following reasons.
By an examination of the monthly distribution of the
comets according to the dates of their perihelion passages — that
is, of the times when they occupy that portion of their orbit in
the vicinity of which they are most visible — Arago has found
that out of 226 comets the distribution is 130 for the winter
months, and 9G only for the summer months. Out of
301 apparitions of comets which have been catalogued we find
that 165 have passed their perihelion between September and
March. The proportion is thus in both cases about fifty-four
or fifty-five to one hundred. Now, such a difference can only
arise from one cause, and that a very natural one — viz. that
the nights in winter are long and favourable to a lengthened
observation of a much greater portion of the heavens, whilst in
summer, the nights being much shorter, and further diminished
by the long twilight, it necessarily follows that a greater
number of comets escape detection. The difference is one-
seventh, a fraction which may be added to the preceding
number, to include comets which in this way escape observa-
tion, making in all 4,228 comets.
To this reason for increasing the number may be added
the fact that observers and comet-seekers are more numerous
in the northern than in the southern hemisphere of the earth,
and in consequence a certain number of comets whose orbits
are so inclined that they are only visible about the time of
their perihelion in the southern hemisphere are unobserved.
But it is clear that we have already partially allowed for this,
by making the number of comets which pass their perihelion
in the summer equal to those which make their passage in the
winter. In fact, those portions of the heavens which cannot
be examined, on account of the shorter nights in the northern
hemisphere of the earth, are precisely those which at the same
161 M
THE WORLD OF COMETS.
epoch are in view in the southern hemisphere. If the number
of observers were the same in both hemispheres, it is clear
that there would be no correction whatever to be made.
Moreover, it is far from being the fact that all comets
coming within sight of the earth are observed; or that the
astronomers who devote themselves to the laborious search
for new bodies can explore continually every region of the
heavens. There is a great difference between searching
for telescopic comets and small planets; the latter must cut
the ecliptic at some time during their revolution, without
ceasing to be visible, and observers have only to examine a
comparatively small region of the heavens situated north and
south of the ecliptic. Comets appear and disappear in all
regions of the heavens alike, and thus a great number must
escape the researches of astronomers. Again, the weather is
not always favourable ; and if the comet should be one of those
which pass close to our globe, and from the rapidity of its
motion should be visible only for a few weeks or days, a
cloudy sky may very easily veil the whole of the apparition.
The light of the moon is also another obstacle which may cause
a comet to escape the detection of observers. The following
passage from the Qucestiones Naturales of Seneca proves that the
ancients even suspected that comets existed in greater numbers
than the frequency of their apparition seemed to indicate: ' Many
comets,' he says, ' are invisible, because of the far greater bright-
ness of the sun.' Posidonius relates ' that during an eclipse
of the sun a comet became visible which had been hidden
through his vicinity.' *
Thus, already we must reckon the comets by thousands,
* [Mr. Ranyard has remarked a structure upon the photographs of the solar
eclipse of December 12, 1871, which may possibly be a faint, though large
comet near to perihelion. See Monthly Notices of the Royal Astronomical
Society, vol. xxxiv., p. 365 (June 1874).— ED.]
162
THE NUMBER OF COMETS.
confining ourselves to those which have appeared in the course
of 2,000 years, — a minute in the probable duration of the solar
system! But our calculations have reference only to those
which approach the earth near enough to become visible. It
remains to estimate the probable number which traverse our
system at all possible distances, when we shall arrive at
numbers so great that they will justify the expression used
by Kepler. We shall follow as our authorities Lambert and
Arago, modifying according to the state of the facts in the
present day the figures employed by them in their estima-
tion of the number of comets within our system. Lambert
relies for his values upon the elements of the twenty-four
comets of Halley's table. In the first place, he reduces their
number to twenty-one, on account of the two re-appearances
amongst them; in the next, he considers the position of the
perihelia, two of which exceed the orbit of the earth; two are
situated between the earth and Venus, twelve between Venus
and Mercury, and, lastly, six between Mercury and the sun.
These numbers, according to him, are in accord with the
hypothesis that comets are uniformly distributed throughout
the interplanetary spaces. But, we may ask, according to what
law is the number of known cornets found to increase? At
first sight it would seem that it should increase in the ratio of
the spaces included within the spheres of the different planets ;
that is to say, proportionally to the cube of the distance.
But Lambert assumes ' that comets are disposed in such a
manner that they never meet or disturb each other in their
movements. To effect this their orbits must not intersect each
other anywhere; further, these orbits are not to be regarded
as geometric lines, but are to include as much of the sphere of
activity of each comet as will prevent incursions into the
spheres of others, and avoid the disorders which would result
from these incursions.' From this restriction — deduced from
163 M 2
THE WORLD OF COMETS.
the principle of final causes, and which at the present time
seems to us quite unjustifiable, as, observations having proved
the intersection of the orbit, the resulting perturbations are
perfectly possible— the celebrated mathematician reduces the
increase of the number of comets to the ratio of the square of
the distance. The numbers six and seventeen, which in
Halley's table give the comets comprised within the spheres
of Mercury and Venus respectively— numbers which are nearly
in the relation of one to three— in his opinion justified this
hypothesis. Taking, then, as our basis the comet of 1680,
whose perihelion was more than sixty times nearer the sun
than Mercury, Lambert came to the conclusion that the sphere
of the orbit of this planet may contain sixty times sixty, or
3,600 comets. Considering, then, the orbit of Saturn, whose
radius is equal to twenty-four times the radius of the orbit
of Mercury, he multiplies by 600 the preceding number, and
finds there would be more than two millions of comets moving
within this sphere.
If at the present time we were to make a similar calcula-
tion, it would be, in the first place, necessary to double our
fundamental number, since in addition to the comet of 1680
we have the comet of 1843, whose perihelion distance is
equally small, and in the next place, we should have to extend
the limits so as to include the planet Neptune. Under these
conditions we should find not less than 45,500,000 comets !
Arago, after discussing the elements of the comets con-
tained in the catalogues at the time when he wrote his
Astronomie Populaire, adopted the same fundamental principle
as Lambert ; that is to say, he assumed the uniform distribution
of comets within the space included by the solar system. At all
events, he says, ' no physical reason can be advanced for
assuming the contrary.' But, with reason, he rejects Lam-
bert's second principle, which restricts the increase of comets
164
THE NUMBER OF COMETS.
to the ratio of the squares of the distances; he adopts the
hypothesis that the increase is as the cube. Now, in the
catalogue of 1853 there are thirty-seven perihelia whose
distances from the sun are less than the radius of the orbit of
Mercury. It will be necessary, therefore, he says, to make
this proportion:
I3 is to 783 as 37 is to the number required ;
or, performing the operations indicated,
1 is to 474,552 as 37 is to 17,558,424.
Thus, within the orbit of Neptune the solar system would
be traversed by seventeen and a half millions of comets. A
similar calculation, now that the number of comets whose
perihelion distances are inferior to the distance of Mercury
amounts to forty-three, would give more than twenty millions
of comets.
We are unwilling to leave the subject without appending
to the values we have just recorded a few reflections which
may enable the reader to better appreciate their import.
Everyone will readily comprehend that the question is
indeterminate, and that its approximate solution can never do
more than assign an inferior limit of the number required.
Admitting the uniform distribution of comets in space as a
probable fact, it will be seen at once that the result of the
calculation will depend solely upon our fundamental number,
as, for example, on the number of comets that pass between
the sun and Mercury. Now, the number we have taken is
evidently much inferior to the number of comets which in
reality have penetrated this region of space in historic times.
If for 2,000 years observation and research had been carried
on as during the last two centuries, what numbers of distinct
comets would not our catalogues contain! More than that, in
each century the number, not counting re -apparitions, would
105
THE WORLD OF COMETS.
continue to increase, and the preceding values would rise in
a similar proportion.
Besides, why limit the space by the orbit of Neptune? Is
it not evident that the sphere of the comets which have gravi-
tated at least once round the sun must extend to all those
regions of the heavens where the attraction of his mass prepon-
o
derates? Let us suppose that stars of the first magnitude have
masses nearly equal, on the average, to that of the sun, and
that they are nearly equally distributed over the sphere whose
radius is equal to the mean distance of Alpha Centauri; the
action of the sun would extend to the half of this distance ;
that is, to about 100,000 times the radius of the earth's orbit.
Every comet penetrating within this distance would fall under
the dominion of our system and gravitate around its central
luminary in an orbit whose elements would depend upon its
initial velocity.
If we extend to a sphere of these dimensions the calculation
we have made for a sphere extending to Neptune, does the
reader foresee in what enormous proportion the results of our
previous calculation will be multiplied? It would be in the
proportion of the cube of 30 to the cube of 100,000 ; that is
to say, it would be multiplied by thirty-seven thousand millions.
So that instead of obtaining the already great number of twenty
millions of comets we should arrive at the stupendous number
of 74,000,000,000,000,000, or seventy-four thousand billions
of comets, as the minimum number of those which have each
been submitted for one at least of their periods to the empire
of the sun !
In presence of such considerations the comparison of
Kepler is no longer a metaphor, and we are permitted to say
literally with the great astronomer of the sixteenth century:
1 Comets are as numerous in the heavens as fishes in the ocean.'
166
SECTION II.
COMETS WITH HYPERBOLIC ORBITS.
Do all comets belong to the solar system ? — Orbits which are clearly hyperbolic —
Opinion of Laplace with regard to the rarity of hyperbolic comets — Are there any
comets which really describe parabolas ? — First glance at the origin of comets.
Do all the comets which have been observed up to the present
time belong to the solar system ? Or, as we have already
suggested, are there comets which visit the sun but once, and
which before penetrating to the sphere of his activity and
submitting to the influence of his attraction were altogether
strangers to .our system ?
Theoretically speaking the reply is not doubtful. A celes-
tial body, describing under the influence of gravitation an orbit
of which the sun is the focus, may move in a parabola, an
ellipse, or an hyperbola. All depends upon its velocity at any
one given point of its course, that is, upon the relation existing
between the velocity ond the intensity of gravitation at that
point. The better to explain this let us take a point whose
distance from the sun is equal to the mean distance of the
earth, and let us suppose the body to have arrived at this point.
For certain velocities, which we may call elliptic or planetary
velocities, the orbit described will be either a circle or an ellipse ;
for a greater velocity (equal to the mean velocity of the earth
multiplied by the number 1..414, that is by the square root of
2), the curve will be a parabola, with endless branches ; for a
167
THE WORLD OF COMETS.
velocity greater still the orbit will be an hyperbola, which also
is a curve with branches extending to infinity.
The question is then reduced to this : Are there any known
comets with parabolic or hyperbolic orbits ? In regard to the
parabolic orbits there may be a doubt, because we may always
suppose that apparently parabolic orbits are really ellipses of
extreme length and considerable eccentricity ; but if there be
orbits of manifestly hyperbolic character, that is to say, whose
eccentricity exceeds unity by an amount greater than we can
attribute to errors of observation, no doubt can exist, because
the curve described cannot be mistaken for a closed or elliptic
orbit. Now, among the comets whose elements have been
calculated there are a certain number whose orbits manifestly
present this character. We give a list of those which, accord-
ing to M. Hoek, merit in this respect a certain amount of con-
fidence : —
Comets ivitli hyperbolic Orbits.
Eccentricity.
. 1-00173
1-00021
Comet 1824
II.
1840
I.
1843
II.
1844
III.
1847
VI.
1849
I.
1849
II.
1853
IV.
1863
VI.
1-00035
1-00035
1-00013
1-00002
1-00071
1-00123
1-00090
Perihelion distance.
. 1-05
. 0-62
. 1-62
. 0-25
. 0-33
. 0-96
. 1-16
. 0-17
1-30
In the catalogue of comets given by Mr. Watson there are
also several comets with hyperbolic orbits — viz. those of 1729,
1771, 1773, 1774, 1806 II., 1826 IL, 1852 II., and 1853 IV.—
whose respective eccentricities are 1*00503, 1-00937, 1*00249,
1-02830, 1-01018, 1-00896, 1-05250, and 1-00123. As the
number of comets catalogued is about 311, it will be seen that
one out of every twenty, or nearly so, is certainly foreign to
the solar system. It is, therefore, possible that a certain
number of the non-periodical comets describe hyperbolas, the
1G8
COMETS WITH HYPERBOLIC ORBITS.
visible portion of which is for us confounded with the arc of a
parabola ; all others would have for their orbits very elongated
ellipses, and thus would be confirmed the hypothesis advanced
by Laplace in the last chapter of his Exposition du Systeme du
Monde : —
' If we connect the formation of comets with that of nebulae,
we may regard the former as small nebulae wandering from one
solar system to another, and formed by the condensation of
the nebulous matter scattered with such profusion throughout
the universe. Comets would thus be in respect to our system
what the aerolites are to the earth, to which they appear to be
foreign. When these bodies become visible to us they offer so
strong a resemblance to nebulae that they are frequently mis-
taken for them, and it is only by their motion, or by our know-
ledge of all the nebulas belonging to the part of the heavens in
which they are moving, that we are able to distinguish them.'
In the celebrated passage which closes the Exposition du
Systeme du Monde, in which the illustrious mathematician ex-
presses his views on the formation of the planets and the
sun, he has added this remark in regard to comets : ' We see
' O
that when they reach those regions of space in which the in-
fluence of the sun is predominant he compels them to describe
either elliptic or hyperbolic orbits. But there being no reason
why they should have a velocity in one direction rather than
in any other, all directions are equally likely, and they may
move indifferently in any direction and at any inclination to
the ecliptic, which is in accord with what has been observed.'
Laplace next examines the cause of the rarity of hyper-
bolic orbits, and, in fact, at the time when he wrote no orbits
were known that could with certainty be said to possess this
character, and he concludes that it is owing to the conditions
of visibility, by which it happens that comets are observable
only when their perihelion distances are inconsiderable. ' We
109
THE WORLD OF COMETS.
may imagine,' he proceeds, ' that, to approach so near the sun,
their velocity at the moment of their entrance into the sphere
of his activity must have an amount and direction comprised
within very narrow limits. Determining by the theory of proba-
bilities the ratio of the chance that, within these limits, the orbit
should be an appreciable hyperbola, to the chance that it should
be an orbit which could be confounded with a parabola, I find
that the odds are at least six thousand to one that a nebula pene-
trating into the sphere of the sun's activity in such a manner
as to admit of its being observed should describe either a very
long ellipse or an hyperbola, which through the magnitude of
its greater axis would sensibly coincide with a parabola in the
part of its orbit where it is observed. It is, therefore, not
surprising,' concludes Laplace, ' that up to the present time
hyperbolic movements have not been recognised.' But, in the
last three-quarters of a century, the progress of theoretical and
practical astronomy, by rendering the determination of come-
tary orbits more exact, has altered the chances whose ratio was
calculated by Laplace.
As regards truly parabolic orbits they can only be rare
exceptions. If we imagine a parabolic comet entering into the
sphere of the planetary system, the least perturbation modify-
ing its velocity in one direction or the other will transform the
orbit either into an hyperbola or an ellipse, either casting the
comet thenceforth from our system or, on the contrary, com-
pelling it to become a periodical satellite of the sun.*
* [It is especially to be noticed that while for an elliptic orbit, the eccentricity
may have any value less than 1, and for a hyperbolic orbit any value greater
than 1, yet for a parabolic orbit the eccentricity must be exactly equal to 1 : so
that parabolic orbits are infinitely less likely to occur than elliptic or hyperbolic
orbits, as the least deviation from the exact value, 1, would make the orbit fall
within the two latter categories. Of course no orbit is accurately an ellipse,
parabola or hyperbola, as the planetary perturbations must produce some modi-
fication of form ; but ignoring these deviations, an absolutely 'parabolic orbit is
all but an impossibility. — ED.]
J70
SECTION III.
REMARKS ON THE ORIGIN OF COMETS.
Have all the known comets of the solar world always belonged to it ? — Probable
modification of their original orbits through the planetary perturbations — Cause of
the gradual diminution of the periods of certain comets.
THE origin of comets is a question equally interesting and
difficult.
On comparing all the orbits that have been calculated we
find that they pass by almost imperceptible gradations from
comets of short period to comets of periods of immense length,
and thence to others the major axes of which are of infinite
dimensions. If we suppose the latter to be strangers to our
solar system, have the former, we may ask, always formed a
part of it ? In which case why should periodical comets in the
elements of their orbits and their physical constitution differ
so essentially from planets ? Why do they cut the plane of the
ecliptic at all inclinations, and why are their movements some-
times direct and sometimes retrograde ? Why are their masses #tl
so small, and why do they exhibit such vaporous appearances,
such rapid changes of aspect, and the phenomenon of tails ?
On the other hand, if comets are all of extra-solar origin,
why have not all cometary orbits a major axis equal at least to
the radius of the sphere of the sun's activity ?
The reply to the first questions would be difficult on the
hypothesis of comets having the same origin as the planets.
171
THE WORLD OF COMETS.
If on the contrary, we admit that comets come from the
depths of the sidereal universe, the comparatively slight eccen-
tricity of certain orbits can be explained by the modifying
action of the planetary masses upon the original orbit. We
have seen that perturbations exerted in the opposite
direction have been able to eject certain comets from the
system, and that the disappearance of some periodical comets
is thus explained. Moreover, independently of this cause,
there is another which also depends upon the insignificance of
cometary masses. We refer to the cause which diminishes con-
tinually the periods of revolution of the comets of Encke and
D' Arrest. Whether it arises from a resisting medium or a re-
pulsive force radiating from the sun, the result is the same — a
progressive diminution of the mean distances of the two comets
from the sun, and the probability that in the lapse of time
these two vaporous masses will become blended with the solar
globe itself.
In our second volume, which will form a continuation of
the present work, and which will be devoted to the subject
of Shooting Stars, we shall give new proofs with regard to the
origin of comets. It will be there seen how they arrive from
all points of space in their peregrinations from world to world,
and wheel around the sun like moths about the flame of a
candle, some to be there consumed and feed the incandescent
ruler of our .system, others to be dispersed in long trains and
shed the dust of their atoms in the interplanetary spaces. The
shooting stars, which vary the sublime, but never-changing,
spectacle of the starry nights of the earth, are but fragments
of dispersed comets. Who knows but that the incessant
rencontres of the planets with these cosmical atoms may be a
means of increasing the planetary masses ? Who knows but
that comets play an important part in the formation and
evolution of planetary systems? This point, insignificant as
172
REMARKS ON THE ORIGIN OF COMETS.
it may appear, in contrast with our enormous planet on the
one side and our brief existence on the other, may in the
course of time exercise a considerable influence upon the
earth's mass. For its operation, this influence has time-
millions of years, — and the nebulous matter ' scattered,' as
Laplace has said, ' with such profusion throughout the
universe.'
173
SECTION IV.
SYSTEMS OF COMETS.
Comets which have or seem to have a common origin — Double comets — Systems of
comets according to M. Hoek — Distribution of aphelia over the celestial vault ;
region of the heavens particularly rich in aphelia.
WHEN, in accordance with the actual facts of science, we
endeavour to form an idea of the constitution of the visi-
ble universe, we see that the celestial bodies which compose
this whole are everywhere distributed into groups and associa-
tions united by the common bond of universal gravitation.
There are the planetary systems. In the centre of each
group is a star or central sun, whose preponderating mass
retains near him, circulating in regular orbits, other stars or
planets, to which this central sun distributes heat and light.
Our planetary system is the type of associations of this kind.
There are the stellar systems, groups of two, three, or more
suns gravitating about one another, probably in accordance
with the same laws. v These systems are themselves the
elements of greater associations, which, like the resolvable
nebulas known under the name of stellar masses, are composed
of myriads of suns. The Milky Way is one of the most splen-
did examples of these immense agglomerations.
In certain regions of the heavens the nebulas are themselves
to all appearance grouped into systems, so that the general
plan of the universe is one vast synthesis of associations of
174
SYSTEMS OF COMETS.
different orders encompassing each other without end. Nor
can any individual star escape the necessity of forming a part
of one of these groups.
Are there likewise systems of comets ?
It is certain, in the first place, that there are some comets
which belong to the solar system. Originally strangers,
they have become drawn into it by the action of the planetary
masses, and have since contributed to form an integral part
of the group. We have seen that it is possible for comets,
through the effect of perturbations, to escape from the power
of the sun's attraction; others, on the contrary, owing to the
insignificance of their masses, unable to resist the causes that
tend to precipitate them into the focus of their movement,
may possibly become blended with the central mass ; or per-
haps, shattered and scattered throughout the interplanetary
spaces by the successive perturbations of the planets, they
may constitute a sort of resisting medium, the elements of
which in the course of time may be a source of increase to the
planetary masses themselves.
Besides, we are already in a position to answer the ques-
tion. We have seen Biela's comet divide into two; and the
twin bodies into which it separated, performing their voyage
in concert, may be said to constitute an embryo cometary
system. The comet observed by M. Liais in 1860 was an
example of another kind, since, if the two comets of which it is
formed should withdraw from the sun, and, still maintaining
their relative position, should leave the system, they would
constitute in space a group of two independent comets.
But are all the other comets — I mean the non-periodical
comets which describe parabolas or hyperbolas — are they to
be regarded as independent voyagers journeying from one
solar system to another, and never staying their vagrant
course? Are there not amongst these some which move
175
THE WORLD OF COMETS.
in groups and make the circuit of their long orbits in com-
pany together?
This question appears capable of direct solution through
the researches of a Dutch astronomer, M. Hoek. By com-
paring and studying the elements of different comets M. Hoek
has discovered that several of their number appear to have had
a common origin, and that before entering the sphere of the
sun's attraction they formed groups or systems, in proof
of which he shows that at some former epoch these bodies were
near together, and had each an initial movement in the same
direction and of the same velocity. Moreover, in his opinion,
comets of elliptic or periodic orbits form the exception, the
immense majority of comets moving in curves with endless
branches. Arriving singly or in groups from the sidereal
depths, they enter our system, sent thither by some star from
which they have receded so far as to be beyond the pre-
ponderance of its attraction, and to fall temporarily under the
attraction of our own sun. But in what manner has M. Hoek
discovered that certain comets have emanated from the same
focus and have probably a common origin ?
To solve this difficult question the Dutch astronomer has
compared the elements of the comets which are determined with
sufficient accuracy to admit of comparison, those — for example,
of the comets calculated since 1556. He has determined the
positions of their aphelia, collecting first in a separate group
the comets whose apparitions were not separated more than
ten years, and whose aphelia were included within a circle of
about ten degrees diameter. And further, he has investigated
whether the orbits of comets thus grouped three and three
or in greater numbers have not points of intersection in
common.
Let us, following M. Hoek, take an example, selecting
in the first place the comets of 1672, 1677, and 1683, and in
176
SYSTEMS OF COMETS.
the next place the comets 1860 III., 1863 I., and 1863 VI.
The positions of the aphelia of these six comets are as
follows : —
Longitudes. Latitudes.
1672 .
1677 .
1683 .
1860 III.
1863 1.
1863 VI.
279-4
286-4
290-8
303-1
313-2
313-9
69-4
75-7
83-0
73-2
73-9
76-4
Now ten degrees of longitude, at a latitude of 73°, represent
an angular distance of 3 J°, so that the differences of longitude
measured upon the arc of a great circle are equivalent in each
group to a little more than three degrees. This of itself is a
remarkable coincidence. But if we investigate the points of
intersection of the different orbits a still more surprising co-
incidence appears, for we find that these points are grouped
together in a region of the heavens the extent of which is
not more than two degrees in diameter, and whicH has its centre
at about 319° of longitude, and 78° of south latitude. By draw-
ing a straight line joining the sun and y Hydrae we obtain nearly
the common intersection of the orbits of the last five comets.
On calculating the distances between the comets and the
sun at different epochs in past ages M. Hoek has obtained the
results which are given in the following tables, the unit of
distance being the mean radius of the terrestrial orbit: —
Date.
Distances from sun.
Comet 1677,
1683.
Uomet i860 iij
I. 1803 1.
1863 V
573-9 600
601-9
757-0 600
600-4
600-2
837-8
500
502-2
1020-9
500
500-6
500-4
1076-5
400
402-4
1259-6
400
400-7
400-5
1286-9
300
302-9
1470-0
300
300-9
300-8
1464-7
200
203-6
1647-8
200
201-1
201-2
1602-0
100
105-1
1785-1
100
101-8
102-1
1833-7
50
52-8
533
18536
20
24-4
25-5
1858-0
10
15-9
17-4
177
N
THE WORLD OF COMETS.
These tables show that the further back we go the more
nearly the comets of 1677 and 1683, and the three comets of
1860 III., 1863 I., and 1863 VI., are found respectively to
approach each other. Have they started simultaneously on
their course, or has each had a separate epoch of departure ?
M. Hoek gives no opinion in favour of either of these hypo-
theses. Only, he shows that the extremely small difference of
26 inches per second between the initial velocities of the comets
of 1677 and 1860 (supposing them to have started together
from a distance so great as to be practically infinite, i.e. to have
been originally fragments of the same body) would suffice to
produce a difference of 200 years in the times of their arrival
into our system ; it is, therefore, not impossible that the two
comets of 1677 and 1860 may have quitted at the same time
the focus from which they emanated.
Let us take another example from M. Hoek — comets III.
and V. of 1857 and comet III. of 1867. These three comets,
in fact, described orbits with elements so similar, and the inter-
vals separating their apparitions were so short, as to point to
the probability of a common origin. At first M. Hoek only
regarded the two former comets as forming a system, but the
comparison of the third with the other two removed all doubt
from his mind.
Speaking of the two comets III. and V., 1857, M. Hoek
proceeds, ' I did not hesitate to attribute to these two bodies a
common origin, considering the extreme resemblance of all
the elements of their orbits, and the short interval between
their appearance. The comet 1867 III. has just given an un-
expected confirmation to this view. The circle which is the
intersection of its orbit with the sphere passes through almost
the same point of the sky. The planes of the three orbits
intersect therefore in the same line, which is necessarily parallel
to the direction of the initial motion of the comets.'
178
SYSTEMS OF COMETS.
The radiant point of their orbits — that point in which their
planes intersect each other — is situated in the southern hemi-
sphere, upon the confines of the constellation of Piscis Australis.
This cometary system is not the only one. In the first
place, the three comets mentioned above are not the only mem-
bers of the group, to which must be added the following comets :
1596, 1781 I, 1790 III., 1825 I., 1843 II., and 1863 III.,
and even 1785 II., 1818 II., 1845 III. The subjoined table
sums up the conclusions of the learned astronomer : —
I. First system.
II. Second system.
III. Third system.
IV. Fourth system.
V. Fifth system.
VI. Sixth system.
Comets.
1677
1683
1860 III.
1863 I.
1863 VI.
1739
1793 II.
1810
1863 V.
1764
1774
1787
1840 III.
1596
1781 I.
1790 III.
1825 I.
1843 II.
1863 III.
1785 II.
1818 II.
1845 III.
1857 III.
1857 V.
1867 III.
1773
1808 I.
1826 II.
1850 II.
1689
1698
1822 IV.
1850 I.
179
Longitudes and latitudes
of the radiant point.
319°, - 78°-5
267°, - 52C
175°-5, - 46°-5
75°-5, - 51°-7
274°-6, + 38°-7
92° 9, + Oc>6
THE WORLD OF COMETS.
VII. Seventh system.
Comets. Longitudes and latitudes
of the radiant point.
1618 II.
1723
1798 II.
1811 II.
1849 I.
217° 8, + 26°-6
In the preceding section we have already said a few words
on the origin of comets, a question still involved in much ob-
scurity. We here merely quote from the Monthly Notices of
the Royal Astronomical Society* the following summary of the
views to which M. Hoek's researches lead:— 'Every star is
associated with a cometary system of its own ; but owing to
the attraction of planetary or other cosmical matter, these
bodies continually leave their proper primaries, and revolve
either permanently in ellipses, or temporarily in parabolas ir
hyperbolas, round other suns.'
On studying the distribution throughout the celestial
sphere of the aphelia of 190 cometary orbits M. Hoek
discovered a somewhat curious fact. If we suppose a circle
drawn through three points, the respective longitudes of which
are 95°, 169°, and 243°, and the latitudes 0°, 32°, and 0°, the
sector comprised between this circle and the ecliptic will be
found particularly poor in aphelions. Instead of including
fifteen, as it would were the distribution uniform, it contains
only one, that of the comet of 1585, situated at a distance of
three degrees only from the ecliptic. How is this peculiarity to
be explained ? To this question M. Hoek replies, ' If we knew
that the solar system was removing from the point situated in
the middle of that sector, I should be inclined to attribute the
phenomenon to a difficulty comets might experience in over-
taking the sun. But the direction of the solar motion, such as
it was given by Madler's investigations, does not allow of such
* Vol. xxvi., p. 147 (February 1866).
180
SYSTEMS OF COMETS.
an explanation.* Therefore we may ask if the phenomenon is
a real one, and there is in that direction of the heavens a
scarcity of centres of cometary emanations ; or rather, if it
depends on the circumstances under which comets are ordi-
narily detected, the sector in question being so near the part of
the ecliptic occupied by the sun from July to December.' f
The first of these two hypotheses is not in our opinion at
all improbable ; the labours of Sir John Herschel on the distri-
bution of nebulas prove that they are disposed very unequally
in the different regions of the sky. A similar inequality in the
distribution of the nebulous centres from whence the comets
emanate would be a fact of the same kind, and one perhaps not
without physical connexion with the first. If future observa-
tions should establish this connexion, it would add one more
gleam of light to those which astronomy has already thrown
on the constitution of the universe.
* See on this subject two interesting letters from M. Hoek to M. Delaunay.
Comptes rendus de I'Acade'mie des Sciences, 1868, 1.
•f Monthly Notices of the Royal Astronomical Society, vol. xxvi., p. 207. M.
Hoek's other papers are published in vol. xxv., p. 243 (June 1865), vol. xxvi.,
p. 1 (November 1865), and vol. xxviii., p. 129 (March 1868).— ED.
181
SECTION V.
COMETAKY STATISTICS.
Comparison of the elements of cometary orbits— Eccentricities ; numbers of elliptic,
parabolic, and hyperbolic comets — Distribution of comets according to their nodes
and perihelion distances— Equality of the numbers of direct and retrograde orbits.
IF we arrange in the order of date the various apparitions of
comets that have been recorded, and note how these bodies
appear in different regions of the heavens, and how some
pursue a direct and others a retrograde course ; or, better, if we
study their elements in a catalogue, our attention is at once
arrested by the diversity of these elements, which seem con-
nected by no relation.
It may, however, be instructive to examine, by comparing
these materials, whether any law presides over the distri-
bution of comets in time and space. We shall, therefore,
give a rapid resume of the analysis we have made with this
object. We have taken the catalogue published by Mr.
Watson at the end of his work on Theoretical Astronomy as
the basis of our investigation.
In this catalogue, which we reproduce at the end of this
work, we find 279 comets arranged in the order of their
successive apparitions, from the most ancient times to the
commencement of the year 1867; we have ourselves com-
pleted it for the seven following years, including also the first
182
COMETARY STATISTICS.
half of the year 1874; so that the total number of comets in
the catalogue is by this means increased to 311, a number
very inferior, not only to the actual number of comets, but to
the number of those which have received mention in history.
Pingre, in his Cometographie, enumerates 400 comets whose
apparition he considers almost certain, and many others which
he has registered as doubtful. His list, however, ends with
the year 1781. Since that epoch 212 comets have appeared.
The catalogue that we are about to study includes only those
comets whose elements astronomers have found means to
calculate. Up to the end of the sixteenth century these
calculations are in general founded upon observations often-
times uncertain and leaving much to be desired on the score of
accuracy ; since then, under the twofold influence of improved
observation and theory, a greater and steadily increasing
degree of accuracy has been obtained.
Let us first consider the form or geometrical nature of
cometary orbits. This form is determined by the element
eccentricity. If the eccentricity is equal to 1 (unity), the
orbit is a parabola, or an ellipse so elongated as to be indis-
tinguishable from a parabola of the same perihelion distance
and direction of axis. If it be less than 1, the orbit is an
ellipse; in this case the comet is periodical, and the duration
of its revolution round the sun may be more or less approxi-
mately determined. Lastly, if the eccentricity be greater than
1, the orbit is hyperbolic.
This being premised, out of 311 comets in the catalogue
we find that 177 have parabolic orbits, 120 elliptic, and only
fourteen hyperbolic. But these numbers require modification,
because they apply, not to distinct comets, but to all observed
apparitions, and consequently to comets which, having reap-
peared, are included more than once in the enumeration.
Taking into account, then, these multiple apparitions, we have
183
THE WORLD OF COMETS.
in all 264 distinct comets, the orbits of which are thus
distributed : —
Parabolic orbits 177
Elliptic orbits 73
Hyperbolic orbits 14
This proves that of known comets the most numerous are
those which really perform their revolution round the sun,
and, but for unknown perturbations, would remain members of
the solar system. If we confine ourselves to the eighty-seven
comets whose orbits have been really determined, we find that
about one in six are foreign to our system. With respect to
the 177 comets which describe parabolic orbits it is still a
matter of doubt whether in reality they move in very long
ellipses or in hyperbolas differing but little from parabolas.
If the 177 comets which seem to be parabolic were divided
in the same proportion between the really elliptic and decidedly
hyperbolic, we should then find that, out of 264 distinct
comets, the distribution would be as follows: —
222 elliptic orbits, or periodical comets.
42 hyperbolic orbits, or comets foreign to the solar system.
But in respect to elliptic orbits we must remember that,
out of the seventy-three comets whose orbits have been calcu-
lated, nine only belong to comets which have actually returned,
or, what comes to the same thing, which have been observed
on two of their successive revolutions.
Let us now proceed to an element of great importance as
regards the study of the distribution of comets in space, viz.
the inclination of the planes of their orbits. The inclination,
however, does not suffice of itself to determine the nature of
this distribution ; it is necessary to add to it the other elements
which fix the position of the curve traced by the comet in the
plane of its motion; the position of this plane itself being
184
\ N
COMETARY STATISTICS.
Inclinations
between
o o
0 and 10
given, in the first place, by the longitude of the node, and in
the second place by that of the axis of the orbit, or the longi-
tude of the perihelion.
We will begin by the study of the inclinations.
These, as we are aware, vary from 0° to 90°. In other
words, a certain number of comets move in the ecliptic, or
deviate but little from it, and might be called zodiacal comets;
others describe orbits which have a moderate inclination to
that of the earth, and others again move in curves which cut
nearly at right angles the paths pursued by our earth and by
the other planets of the solar system.
The following table, in which distinct comets only are
included, shows this distribution: —
Number
of
comets.
21 1
20 >62
21 J
231
39 >97
35 J
31 1
33 [96
32 J
The inclinations of nine cornets are wanting in this table.
These numbers clearly prove that great inclinations occur
more frequently than small. The comets, it may be observed,
that we have proposed to call zodiacal form only a quarter of
the number of distinct comets that have been catalogued. The
other three-quarters are pretty evenly distributed between the
moderate and great inclinations.
Does not this furnish irrefragable testimony of the extra-
solar origin of a great number of comets, since so great a
divergence exists between the planes in which they move and
the planes of the orbits of the planets? This distinctive
feature appears to us all the more striking, because amongst
185
10
„ 20
20
„ 30
30
„ 40
40
» 50
50
„ 60
60
» 70
70
ii 80
80
, 90
THE WORLD OF COMETS.
the number of comets of small inclination there are many
whose movement is retrograde, a fact which adds another
point of difference to those which distinguish the movements
of these bodies from the movements of the planets.
We now come to the longitudes of the ascending nodes
and those of the perihelia. These will be found in the
following table : —
Longitudes of nodes and of
perihelia comprised between
0 ai
id 30
30 ,
, 60
60 ,
, 90
90
120
120
150
150
180
180
210
210
240
240
270
270
300
300
330
330
360
Number of
Number of
comets.
comets.
Nodes.
Perihelia.
20"|
22 V67
25 J
171
24 j-71
30 J
251
251
25 V72
22 J
21 [60
14 J
24 1
22 >66
20 J
16 1
21 }-66
29j
141
22 S53
17 J
301
22 I 60
8J
The nodes, as we may perceive by comparison with the
table on p. 30, exhibit a greater degree of uniformity in their
distribution than the inclinations. Nevertheless, in the last
quadrant of the circumference of the ecliptic the number of
comets which cross the plane of the earth's orbit, from south
to north, is noticeably smaller than in the other three. As
regards the perihelia, the differences in the different quad-
rants are still less. We have seen that M. Hoek, who has
studied the question closely, has made a comparison of the
opposite points or aphelia of various comets, and has arrived
at the important conclusion that a certain number of these
bodies are united in groups, and that each of these groups
includes comets of probably common origin.
Let us now compare the comets, arranged according to
their respective perihelion distances. We will take as unity
the mean distance of the earth from the sun and divide it into
186
COMETARY STATISTICS.
tenths, each tenth corresponding to 2,320 equatorial radii of
our earth, or about 9,200,000 miles. We shall then find the
perihelia of the 258 distinct comets distributed as follows: —
Perihelion distances
comprised between
V V «
o-i
n
0-2
0-2
0-3
0-3
0-4
0-4
n
0-5
0-5
0-6
0-6
n
0-7
0-7
n
0-8
0-8
n
0-9
0-9
n
1-0
1-0
n
1-1
1-1
n
1-2
1-2
n
1-3
1-3
H
1-4
1-4
n
1-5
1-5
n
2-0
2-0
„
6-0
53«
60
130
Number of
comets.
9
11
22
11
k. 1 Q
29
>iu
20
28
26
25 J
16 1
12
11
5 >66
7
7
15
This table shows that by far the greater number of comets
have their perihelia in the vicinity of the earth, between the
planets Venus and Mars, whose mean distances are 0*723 and
1*524 respectively, the earth's mean distance from the sun
being taken as unity. There are no fewer than 130 within
these limits. Comets, on the contrary, whose perihelion
distances are beyond the orbit of Mars, and even beyond that
of Jupiter, are few in number — but fifteen in all; fifty-three
comets have their perihelia comprised within the mean
distance of Mercury, 0*387, and sixty between the orbits of
Mercury and Venus. But, as we have already said, in our
section upon the number of comets, this distribution is in all
probability apparent only, because, being invisible from the
earth, except in the neighbourhood of their perihelia, comets
which do not make a nearer approach to the sun than the
planet Mars are under very unfavourable conditions for ob-
servation ; unless of exceptional brilliancy they would pass
187
THE WORLD OF COMETS.
unperceived from the earth. Comets which have a perihelion
distance comprised between the orbits of Venus and Mars
are, on the contrary, near enough for observation; but, on
the other hand, their close vicinity to the earth renders
their apparent motion very rapid, and they are only visible
for a brief period. In short, the most likely comets to
be observed are those which pass between the sun and
Venus ; and on the hypothesis of an equal distribution in
space these ought to be the most numerous, regard being had
to the volumes of the spheres in which their perihelion dis-
tances are contained.
Lastly, let us consider the movement of comets. All
comets whose orbits, projected on the ecliptic, are described in
the direction of the earth's movement are direct ; all those which
move in an opposite direction are retrograde. Now, out of
252 distinct comets 129 are retrograde and 123 direct. Their
numbers are, then, nearly equal. How these numbers are
divided between parabolic, elliptic, and hyperbolic orbits the
following table will show : —
Direct comets .
Retrograde comets
Thus, the comets decidedly elliptic seem to show a greater
preference to move in the direction of the planetary move-
ments than comets which are parabolic. However, as the true
nature of the curves described by the latter is a matter of
doubt, it is hardly possible to draw from this circumstance any
certain conclusion as to which direction of movement pre-
dominates. It is a more significant fact that, out of nine
periodical comets of verified return, one alone (Halley's comet)
has a retrograde motion, and that this comet has an aphelion
188
f Parabolic
69")
<^ Elliptic
[ Hyperbolic
44 ^123
10 J
f Paiabolic
981
< Elliptic
^Hyperbolic
27 >129
4j
COMETARY STATISTICS.
distance exceeding the known limits of the planetary system.
If we include the seven other interior periodical comets which
have not yet returned, we find that the movement of fourteen
of them is direct, and that two only describe orbits in a retro-
grade direction. These comparisons become still more striking
when we observe that the inclinations of the nine first comets
are nearly all comprised within the limits of the zodiac. One
of them (Brorsen's) has a larger inclination, of about 29^°,
which is less, however, than the inclinations of three of
the little planets which revolve between Mars and Jupiter.
Tuttle's comet forms the sole exception, its inclination ex-
ceeding 54°. Of the remaining nine periodical interior
comets one alone, the comet 1846 IV. .has the large inclina-
' / o
tion of 85°; two others attain 30°, and six have small
inclinations.
Such are the comparisons that have been suggested to us
by the study of the elements furnished by existing catalogues
of comets. It would be desirable, no doubt, to multiply com-
parisons of the same nature, and to obtain from them further
probable deductions. The work is one that would require
long and minute research, and we have only attempted to give
our readers some idea of these relations. If, instead of
limiting ourselves to points we have considered in this
chapter, we were to include all that has reference to the aspect
and physical constitution of comets, especially since they have
been subjected to rigorous telescopic scrutiny, our field of
research would be greatly enlarged, and our results proportion-
ably increased in number and value. We should, perhaps,
be enabled by a kind of natural classification to distinguish
these bodies into kinds and species and varieties. The
physical explanation of the phenomena which they present
would be rendered easier, because we should not then be
compelled to apply to all a theory which may be suitable
189
THE WORLD OF COMETS.
for some and not for others. This the reader will better
comprehend as he becomes familiar with the subject in this
new aspect, the phenomena it includes, and the explana-
tions suggested. Such is the principal object of the following
chapters.
190
CHAPTER VII.
PHYSICAL AND CHEMICAL CONSTITUTION
OF COMETS.
SECTION I.
COMETS PHYSICALLY CONSIDERED.
The physical or chemical constitution of a celestial body ; nature of the question
involved ; explained by reference to the earth — A cometary problem.
WHAT is meant by the physical or chemical constitution of a
celestial body, or of any luminary whatever, whether star or sun,
planet or moon; or, as we are treating of comets only, what is
meant by the physical or chemical constitution of a comet?
We here have presented for our consideration a question the
nature of which is easily explained and not less easily under-
stood ; but it is one that the best-informed of astronomers would
find it difficult to answer in its full integrity.
By comparison with the bodies that we see on the surface
of the earth and with the terrestrial globe itself, considered as
a whole, we shall proceed to explain what is meant by the
physico-chemical constitution of a comet.
The earth is a globe, more accurately, a spheroid, whose
form and dimensions are perfectly defined and well known, at
all events as far as concerns its solid crust, the atmosphere that
surrounds it. and the rocks and strata near its surface. Bv
it
direct observation we are acquainted with the solid crust to the
depth of many hundred feet, and the atmosphere to a height of
several miles. Induction has supplied us with a knowledge
concerning atmospheric strata to which man has been unable
193 0
THE WORLD OF COMETS.
to ascend, and depths in the earth to which he has not yet
penetrated. The mean density of the earth, its mass and
weight, and the relation of its mass to that of the principal
members of the solar system, are known.
What are comets from these various points of view? Are
they globes similar to our earth, illuminated like it by the
sun, or do they shine by their own light? Have they a solid
or liquid nucleus, surrounded by a vaporous atmosphere, or are
they gaseous masses, collections of particles more or less con-
densed ? Has any certain estimate been formed of their masses,
or the density of the matter of which they are composed ? As
regards their movements we know that they do not differ from
other members of the celestial group of which we form a part,
and that the same universal force, the same laws govern them.
Coming probably from the depths of space, of distinct origin
therefore, and of very different aspect to the planets and their
satellites, we may not apply to both the lines of Ovid:—
-Facies non omnibus una
Nee diversa tamen qualem decet esse sororum.
Comets are, from all these points of view, their movements
alone excepted, conspicuously different from the earth and the
rest of the planets. In physical constitution they appear to
be quite dissimilar— chemically speaking, are they equally un-
like? That is to say, is the matter of which they are composed
formed of unknown elements, or of elements identical with
those of which the planets themselves are constituted ?
All these questions possess a high degree of scientific
interest. Nor are they less important if we view them in their
relation to the superstitious beliefs which for so long a time
made comets formidable to the world — beliefs which, having
changed in form perhaps more than in substance, are still to a cer-
tain extent current even in our enlightened century. Although
not susceptible of proof, the habitability of the planets is a thesis
194
COMETS PHYSICALLY CONSIDERED.
that has long been maintained and is still maintained with
very considerable probability. More than this, in the last cen-
tury it was supposed, and some savants even of our time believe,
that comets have likewise their inhabitants. Are comets indeed
habitable? We are urged by an instinct of invincible curiosity
to put such questions to ourselves ; and if it appears next to
impossible to return positive replies, at least we are not for-
bidden to examine the probability of each. But, if we would
not abandon ourselves to vain and profitless conjectures, it
is clear that we must, in the first place, acquaint ourselves
with what science has to communicate, not respecting this pro-
blem, which may be considered as extra-scientific, but upon the
physical and chemical conditions which observation and ex-
periment show to be compatible with the existence of human
beings, as far as they are known to us.
We shall, therefore, examine what is known of the constitu-
tion of comets at the present day; and we shall begin with the
study of their aspect and external form.
195 o 2
SECTION II.
COMETARY NUCLEI, TAJLS, AND C<X\LE.
Comje and tails— Classification of the ancients according to apparent external'
form ; the twelve kinds of comets described by Pliny — The ' Guest-star ' of the
Chinese — Modern definitions : nucleus, nebulosity or atmosphere ; tails.
WHAT is the distinctive sign of a comet by which it is univer-
sally known, by which it is distinguished from all other celestial,
bodies? Everyone answers at once, it is the train of luminous
vapour, the nebulosity of more or less length, which accom-^
panies it or at least surrounds it ; in other words, the tail and;
the coma
This is what the etymology implies, the word comet signi-
fying long-haired or hairy. Armed with its tail, which appears
brandished in the heavens like an uplifted sword or a flaming
torch, the precursor of some untoward event, a comet is every-
where recognised on the instant of its appearance ; it needs no
passport signed by astronomers to prove its identity. But
should the tail be absent, should no appendage or surrounding
nebulosity distinguish the celestial visitor on its apparition,
for the world at large it is no comet, but simply an ordinary
star like any other.
Nevertheless, there are tailless comets. The comet of
1585 was equal to Jupiter in size, but less brilliant ; its light
was dull. It had neither beard nor tail, and it might have
been compared to the nebula in Cancer (Pingre). Lalande
196*
COMET ARY NUCLEI, TAILS, AJND COALE.
observes that the comets of 1665 II. and 1(,82 ; exhibited
discs as round, clear, and well defined as that of Jupiter him-
self, without tail, beard, or coma? We are here speaking of
comets visible only to the naked eye ; of telescopic comets a
great number are destitute of tail, arid it very often happens
that they are simple nebulosities, in the midst of which a faint
nucleus is but just discernible, sometimes nothing but a luminous
condensation at the centre. Moreover, from the presence or
absence of a tail at one time of the apparition, we cannot infer that
the same is true at another. Thus, the above-mentioned comet of
1682 (no other than Halley's comet), which Cassini observed to
be without tail on August 26, had developed one of 30° in length
by the 29th of the same month. And as regards the comet of
1585, twelve days after its apparition, ' a slender and hardly
perceptible ray was seen to issue from it, a hand's breadth or
more in length.' It likewise often happens that the tail which
has been invisible to the naked eye is readily perceived in the
telescope ; instances of this we shall meet with as we proceed.
All that we have here to bear in mind is, that the distinctive sign
' O
of a comet, astronomically speaking, is not to be sought in the
tail, the coma, or in any of the variable appendages which may
surround the star during its apparition. The elements of its
orbit, its large eccentricity, gr^at inclination, direction (often-
times retrograde), &c., constitute the true points of difference
between a comet and the planets. We have already called
attention to these differences, and need, therefore, only allude
to them here.
It is clear that up to the sixteenth century, before the
employment of the telescope in astronomical observations, the
accounts given of cometary apparitions can refer only to comets
seen by the naked eye. The strange forms of their tails, their
beards and coma?, attracted the attention alike of the multitude
and the learned. The ancients, who have not always clearly
197
THE WORLD OF COMETS.
distinguished them from other luminous meteors, such as bolides
and aurora boreales, applied themselves to a classification of
comets according to their appearance. Pliny has distinguished
not fewer than twelve kinds, which he describes somewhat
obscurely in the following terms : —
* There are,' he observes, * comets properly so called ; they
are fearful by reason of their blood -coloured manes and their
bristling hair pointing upwards. The Bearded (Pogonia?) have
their long hair hanging down like a majestic beard.' (These first
two kinds may be classed together, because they differ only in
the direction of their tails.) 'The Javelin (Acontias), which
seems to dart forward like an arrow; the effect follows with
the utmost speed upon an apparition of this kind. When the
tail is short and pointed it is called the Sword (Xiphias); this
is the palest of all comets ; it shines like a sword, and is with-
out any rays. The Plate or Disc (Disceus) bears a name in
accordance with its figure; it is of an amber colour, and emits
a few rays from the margin only. The Cask (Pitheus) exhi-
bits the figure of a cask, and appears in the midst of a smoky
light. The Horn (Ceratias) has the appearance of a horn, and the
Lamp (Lampadias) that of a burning torch. The Horse (Hippeus)
resembles a horse's mane, agitated violently by a circular or
rather a cylindrical motion. It is also very white, with silver
hair, and so bright that it can scarcely be looked at, exhibiting
the aspect of a deity in human form. Some there are which
are shaggy (hirti, and not Azra, as several have read); these
have the appearance of a fleece, surrounded by a nebulosity.
Finally, the hair of a comet has been seen to assume the form
of a spear.' ;-,
All these denominations are more or less justified by the
diversity of aspect which comets are known to exhibit, and by the
differences observable in their nebulosities and tails ; but they
afford us absolutely no information concerning their physical
198
Pi,. III.
10
11
FORMS OF COMETS ACCORDING TO PLINY,
Taken from the Comttographi<> of Hevclius.
Cometse : 1. Discei, disciformis. — 2. Pithei, doliiformis erectus. — 3. Hippei, equinus barbatus. —
4-5. Lampadife, lampddiformis. — 6. Barbatus.— 7. Cornutus bicuspidatus.— 8. Acontite, faculiformis
lunatus.— 9. Xiphiae, ensiformis.— 10. Longites, hastiformis.— 11. Monstriferus.
COMETARY NUCLEI, TAILS, AND COM.E.
nature. Nor is Pliny's enumeration complete, if we are to
regard as comets the burning torches and beams (faces and
tr'abe's], which he describes separately.
The Chinese, who, fortunately for science, have taken care-
ful note of all cometary apparitions, have given to the tails of
these bodies the very prosaic name of brooms (sui or soui}.*
They likewise acknowledged no comet without a tail. * If
devoid of this appendage,' says Pingre, ' whatever might be its
movement, it was spoken of simply as a star, or the new star, or
the guest-star, from its visiting the provinces and taking up its
abode in different places, as at an inn. Their home was in the
vestibules of the celestial palaces ; there, under an invisible
form, they awaited the order of departure. The order sent,
they became visible and commenced their journey. If whilst
on their way they put forth a tail, the star was said to have
become a comet. 'f
* Comets are called in Chinese ' broom stars,' a name derived from the form
of their tails. As a rule the records make no distinct mention of the nucleus,
and the constellations indicated are generally those over which the tail extended.
Thus, in describing the march of the comet of 1301, the text of the records runs
as follows: ' It swept the star Thien-ki, the Sankoung, &c.' (Biot and Stanislas
Julien, Comptes rendus de V Academic des Sciences, 1842, tome ii. p. 953.^
f This passage will be better understood if we extract from the same author
a second paragraph, in which he explains ' the foolish and singular idea that the
Chinese had formed of the heavens.. Tie heavens were, according to them, a
vast republic, a great empire, composed of kingdoms and provinces ; these pro-
vinces were the constellations ; there was decided all that would happen for good
or ill to the great terrestrial empire, that is, to China. The planets were the
administrators or superintendents of the celestial republic, the stars were their
ministers, and the comets their couriers or messengers. The planets sent their
messengers from time to time to visit the provinces for the purpose of restoring
or maintaining order ; but all that was done in the heavens above was either
the cause or the forerunner of what was to happen here below.'
• We confess that the ideas of the Chinese appear to us hardly more foolish
than the extravagant conceptions of the Europeans in the times of the ancients
and in the Middle Ages; they, at all events, give evidence of a higher idea of
the disposition of the universe. Nor would it be difficult to find amongst our con-
temporaries individuals whose views concerning the government of the world
differ in no essential respect from those of the Chinese.
199
THE WORLD OF COMETS.
But let us return to the definitions accepted by modern
astronomers.
A comet consists, generally speaking, of what are invariably
termed the head and the tj.il.
The head is composed of the star ; that is to say, of the
nucleus or luminous point in which the brightest light of the
star is concentrated, and of the surrounding nebulosity, coma,
or atmosphere. All comets do not exhibit a nucleus ; but those
which appear as simple nebulosities of vaporous appearance
are generally telescopic comets. The head of a comet visible
to the naked eye is always bright and star-like.
When the nebulosity is of nearly circular form, oval or
sometimes irregular — which may arise either from its real con-
figuration or from an effect of perspective — and is devoid of
any prolongation or train, the cornet is said to have no tail, this
denomination being reserved for the luminous train, sometimes
of no great length, sometimes of immense extent, which escapes
from the head in a direction nearly always opposite to that of
the sun at the time of observation. It sometimes happens that
the train is directed towards the sun, or makes a certain angle
with the line joining the head and the sun; it was then called
by the ancient astronomers the beard of the comet, an ex-
pression now discarded. At the present day every luminous
appendage or train of vaporous appearance is spoken of as
a tail.
[I may tere mention that M. E. Hint's ' Catalogue des Cometes observees en
Chine depuis Tan 1230 a 1'an 1640 de notre ere,' forms a supplement to the
Connaissance des Temps for 1846; and that in 1871 the late Mr. John Wil-
liams published ' Observations of Comets, from B.C. 611 to A.D. 1640, extracted
from the Chinese Annals,' which contains a catalogue of the whole of the obser-
vations of the comets recorded in the Encyclopaedia of Ma Twan Lin, and in the
historical work called the She Ke. The catalogue of M. Biot gives notices of
9M comets, and that of Mr. Williams of 373.— ED.]
200
SECTION III.
COMETS DEVOID OF NUCLEUS AND TAIL.
Gradual condensation of nebulous matter at the centre — Imperceptible transition
from comets without apparent tails to the immense luminous trains of great
historic comets.
LET us before proceeding further make a few general re-
marks on the heads and tails of comets. The remaining
sections of the chapter we will devote to a more complete
examination of their structure.
Fig. 21. — Cometary nebulosities; central condensation; absence of tail and nucleus.
Since a systematic search has been made for comets, and
powerful instruments have been employed, the number of those
discovered has, as might be expected, considerably increased ;
but the majority are telescopic comets, and amongst them are
201
THE WORLD OF COMETS.
many nebulosities devoid of nucleus. This fact had been
already ascertained by Sir William Herschel in 1807. ' Out of
sixteen telescopic comets that I have examined, fourteen,' he
observes, ' exhibited nothing remarkable at their centres.'
The following are some examples of comets which were
simple nebulosities, and apparently without tail or nucleus.
Encke's comet, observed by Mr. J. Tebbutt, June 24, 1865:
' The comet,' he observes, ' was about two minutes in diameter,
Fig. 22. — Encke's Comet according to Mr. Carpenter.
faint, and without the slightest condensation of light in the
centre.' In October 1871 the same comet presented, accord-
ing to Mr. Hind, when first observed, the aspect of a faint and
nearly round nebulosity, without any condensation of its
parts. But on the 9th of November the same comet exhi-
bited an appearance anything but globular. According to
Mr. Carpenter the nebulosity had expanded like a fan, the
apex of which was the most brilliant part ; but there was no
nucleus. The comet discovered on July 12, 1870, by Ml
202
COMETS .DEVOID OF NUCLEUS AND TAIL.
"Winfiecfee was similar iri appearance, and is described 'as " a
round nebulosity, of moderate brilliancy, and of 2^ minutes in
diameter.
The following is another instance in which the trace of a
brilliant nucleus is just discernible. We refer to Brorsen's
comet,. 'observed at Marseilles, on September 1, 1873, by M,
Stephan, who thus describes it: ' Nebulosity ovoid, diffuse, and
exceedingly faint, with a trace of condensation towards its
centre.' And likewise Winnecke's comet, seen in April and May
1869: 'It is a faint nebuldus patch of some little size,' says Mr.
Fig. 23. — Encke's Comet, December 3, 1871, according to Mr. H. Cooper Key.
Hale Wortham, 'appearing occasionally to brighten somewhat
to a centre.' According to Father Perry, 'there seems to be a
slight condensation towards the centre, but no decided nucleus.'
However, we must not forget that the absence of a nucleus
may proceed either from the distance of the comet rendering a
very slight condensation invisible, or from the position of the
comet relatively to the sun. If the nucleus shines by a light
which is not its own, its light would increase as the comet
draws near to its perihelion. And we see. in fact, that in fig.
203
THE WORLD OF COMETS.
17, Encke's comet exhibits a visible condensation, while in
no-. 23 it has a brilliant and defined nucleus. In like manner
n
Brorsen's comet, observed in October 1873, showed con-
siderable condensation about the centre. On its apparition in
1868 the brightest portion was very eccentric, and there were
three or four centres of condensation or brilliant nuclei. (See
fig. 18, p. 120.)
The comet of 1867, II., telescopically observed by Mr. Hug-
gins, ' appeared to consist of a slightly oval coma, surrounding a
minute and not very bright nucleus.' This bright point was
not central, but near to the following (eastern) edge of the
coma. The double comet of Biela, as we shall presently see,
possesses a well-defined luminous nucleus in the centre of each of
the nebulosities which compose its two parts. The same fact is
observable in respect to other telescopic comets. In May 1873
Tempe!' s comet exhibited a head of oval form, with a central
nucleus about as bright as a star of the 12th or 13th magni-
tude. Faye's comet, seen at Marseilles, in September of the
same year, although extremely faint, had a small sharply-
defined nucleus, which enabled it to be easily observed. Lastly,
the comet of 1873, IV., discovered by M. P. Henry at the
Observatory of Paris, was round, very brilliant, nearly visible
to the naked eye, and had a central condensation. It is shown
under this aspect in the left hand drawing of fig. 32.
In some comets, as we have seen in the preceding section,
the nuclei have been equal in brilliancy to Jupiter himself;
others that we have yet to mention have even exceeded him in
the brilliancy of their light. Between simple nebulosities,
therefore, devoid of nucleus or luminous condensation, and
those comets which have surpassed in lustre the most brilliant
of the planets, there is no distinct line of demarcation. The
transition from the one extreme to the other is imperceptible. We
shall find a similar gradation in respect to cometary tails, from
204
COMETS DEVOID OF NUCLEUS AND TAIL.
the comets destitute of tail, that we have just described, from
hardly visible traces of these appendages in telescopic comets,
to the immense luminous trains of the great comets of 1680,
1769, 1811, 1843, 1858, &c., which during their apparition
swept the heavens. These differences of aspect the reader
will be enabled to follow by the aid of our engravings.
203
SECTION IV.
DIRECTION OF THE TAILS OF COMETS.
Direction of the tail opposite to the sun ; discovered by Apian ; the Chinese astro-
nomers were acquainted with this law— Deviations in some comets— Variable
aspect of the tail according to the relative positions of the comet, the earth, and
the sun.
IN respect to the direction of cometary tails let us call
attention to an important point — to a general phenomenon
which was remarked by the ancients in the very earliest
times. Seneca refers to it in the following line: —
Comas radios solis effugiunt.
The comce of comets fly the rays of the sun. According to
Edward Biot the Chinese astronomers had observed, since the
year 837, this constant direction of cometary tails from the sun.
'In Europe,' says Lalaride, ' Apian was the first to perceive
that the tails of comets were always opposite to the sun ; this
rule was afterwards confirmed by Gemma Frisius, Cornelius
Gemma, Fracastoro. and Cardan. Nevertheless, Tycho Brahe
did not believe it to be very general or well demonstrated ;
but the fact itself is beyond a doubt.'
Pingre observes with truth that the direction of the tail
is not always strictly opposite to the sun. He instances the
comet of 1577, whose tail was deflected as much as 21°
towards the south, and the great comet of 1680, when the
20$
DIRECTION OF THE TAILS OF COMETS.
deflection was about 4j°. On both these occasions, however,
the comet and the earth occupied the same relative positions in
the heavens. The deviation is less in proportion as the tail is
more inclined to the orbit ; viz., considering only the portion
of the tail in the neighbourhood of the nucleus, the deviation
is less in proportion as the comet draws near to its perihelion ;
and it takes place towards the region of the heavens last
quitted by the comet in its course.
It results, therefore, from this law that the tail of a
comet sometimes follows and sometimes precedes that body
in its course. It follows the cornet before the perihelion
1 Fig. 24. — General direction of cometary tails.
passage, and precedes it when the perihelion has been passed..
Moreover, tails have very frequently a more or less considerable
degree of curvature, and this curvature appears more marked
in proportion as the earth is removed from the orbit of the
comet. If the earth be situated in the plane of the comet's
orbit, the curvature is nil* and the tail is rectilinear, or at
* The two tails of the great comet of 1861 at first appeared to offer an
exception to this law. On the 30th of June, on which day the earth passed
through the plane of the comet's orbit, the two tails, projected the one upon
the other, appeared to form but one, wider in the first third of its length, reckon-
ing from the nucleus ; but both were rectilinear. But M. Valz and Mr. Bond,
from observations made by the former and by Father Secchi, discovered, as they
believed, that the two tails presented a slight deviation to the east of the plane of
207
THE WORLD OF COMETS.
least appears so; this is no doubt an effect of perspective, and
the curvature then takes place in the plane of the orbit. It
is more marked in those portions of the tail that are furthest
from the nucleus; from which it follows that if we draw
radii vectores from the sun to the several positions of the
comet, the tail always presents its convex side to these lines,
as may be seen in fig. 24.
There is yet another conclusion to be drawn from these
facts, which is, that if the earth occupies such a position with
reference to the comet and the sun that the comet is in
opposition to the latter, its tail, being likewise opposite to the
sun, is situated behind the nucleus, and is consequently in-
visible. It is then only the breadth of the tail that is seen, and
it appears to surround the nucleus as a coma, thus increasing
only the extent of the cometary atmosphere. This fact may .
serve to explain the absence of tails in some comets, which,
from their nearness to the earth, we should have expected to
have been so provided.
the orbit. This would render still more difficult the theory of the formation of
tails. But, if we adopt the conclusions of M. Faye in this respect, the deviation
existed in appearance only, and this difficulty would be removed. This is a
point well deserving the attentive consideration of all future observers of
cometary phenomena.
Comets with double tails, one of which is opposite to the sun and the other
directed towards that luminary, appear likewise to follow the law stated above.
Olbers says of the comet of 1823 : ' On the 23rd of January the earth passed
through the orbit of the comet ; on this day not the least deviation could be
discerned between the direction of the abnormal tail and the prolonged axis of
the other tail.' ' Thus,' says M. Faye, in citing this passage, ' the two tails of
the comet were projected, each on the prolongation of the other, which shows
that tails directed towards the sun have, like the others, their axes situated in
the plane of their orbit.'
208
SECTION V.
NUMBER OF TAILS.
Double tails of comets ; comets of 1823, 1850, and 1851 — Tails multiple, fan-shaped,
rectilinear, curved — Variable number of tails belonging to the same comet ; comets
of Donati, of 1861 and of Che'seaux.
GENERALLY a comet has but one tail, which varies considerably
in form or size, or, at all events, appears to do so. Sometimes
these changes take place very rapidly, but still, as a rule, the
tail consists of one luminous train. Nevertheless, examples
may be adduced of double and even multiple tails. The
comets of 1807 and 1843 were furnished with double tails,
or, what comes to the same thing, single tails formed of two
branches of very unequal length. It was the same with the
comet of 1823, about which Arago gives the following
details : —
'On the 23rd of January, 1824, the comet, in addition to
its ordinary tail opposite to the sun, had another which was
directed towards the sun, so that it resembled somewhat the
great nebula of Andromeda. The first tail appeared to include
a space of about 5°, but the length of the second was scarcely
4°. Their axes formed between them a very obtuse angle of
nearly 180° (fig. 25). In the close vicinity of the comet the
new tail was hardly to be seen. Its maximum brightness
occurred at a distance of 2° from the nucleus. During the
200 p
THE -WORLD OF COMETS.
first few days in February the tail opposite to the sun was
alone visible ; the other had disappeared, or had become so
faint that the best telescopes in the clearest weather failed to
show any trace of it.'
The comets of 1850, L, and 1851, IV. (figs. 26 and 27),
exhibited the same phenomenon of two unequal tails, the
shorter of which was directed towards the sun.
Fi'r. 25. — Double tail of the comet
of 1823.
Fig. 26. — Double tail of the comet
of I860.'
The observations and the drawings of Messier show that
the great comet of 1769 had, if not a multiple tail, at least
lateral jets of light, resembling secondary
tails, proceeding from the nucleus, but
much smaller and less extended than the
principal tail, and making unequal angles
with the latter : all the tails were recti-
linear.
Donati's comet exhibited, in 1858, a
similar peculiarity. In addition to the
principal tail, remarkable for its extent,
its curvature, and brilliancy, there ap-
peared first one and then two luminous trains, much fainter, ap-
parently rectilinear, and nearly tangential to the limiting curves
The figures of Plates VII. and IX. give a
210
Fig. 27-— Comet of 18ol.
of the great tail.
NUMBER OF TAILS.
very correct idea of this phenomenon, which was observed in
Europe by Schwabe (of Dessau) from the llth of September,
and then by Mr. Hind at London, and by Winnecke and Struve
at Pulkowa. In America the secondary tails of the comet
were studied and drawn with the utmost care by Professor Bond
of the Observatory of Harvard College. On following the
development of these remarkable appendages by means of the
beautiful plates in the great work* which the American
astronomer has devoted to this cornet, we obtain the following
resume of the changes observed : —
On September 27 a slender rectilinear tail was first per-
Fig. 28. — Sextuple tail of the comet of 1744, according to Cheseaux.
ceived, in part veiled by the principal tail, and nearly of the
same length ; it seemed to be tangential to the concave portion
of the curve. There was no change on the 28th, but on the
29th it began to approach the nucleus. On the 30th it was
* [The work forma Vol. III. of the Annals of the Observatory of Harvard
College (18G2). The reader will find in it almost everything that is known
about the great, comet of 1858, and the plates are so numerous and excellent that
all the changes of form and appearance that the comet underwent, both as
regards its tails and nucleus, can be easily followed. — ED.]
211 r 2
THE WORLD OF COMETS.
hardly visible, but on the following days up to October 3 it
became somewhat brighter ; it was then half as long again as
the principal tails On the 4th of the same month a second
rectilinear tail, not so long as the former, made its appear-
ance, forming with it an angle apparently equal to that
enclosed by the two limiting curves of the principal tail at
their point of departure from the nucleus. On the 5th the
longer was also the brighter. On the 6th, 7th, and 8th of
October the longer of the secondary tails alone was seen ; but
on the 9th the second was seen, and, as it proved, for the last
time. The convexity of the principal tail at this date became
more marked, and the longer of the rectilinear tails, which
had never ceased to form a tangent to the principal tail, was
itself somewhat curved near its base, so that, if continued in
a straight line, it would no longer have terminated in the
nucleus of the comet. These appearances allow us to concede
to Donati's comet a triple tail.
In the last century a comet was observed whose tail, which
was fan-shaped, presented six distinct branches. This is the
famous comet of 1744, known as Cheseaux's comet. Fig. 28
represents, according to the drawing of this astronomer, the
sextuple tail in question. On March 8 its remarkable form was
most observable. The six divergent branches of the tail pro-
ceeded from the nucleus as luminous curves, the outer radii of
which included an angle of about 60°, the longest being towards
the concave portion. Cheseaux saw the comet rise before the
sun, and its large fan appeared above the horizon before the
nucleus itself was visible. This curious phenomenon was
sketched by Cheseaux at Lausanne, and from his original
drawing we have designed Plate V.
Nearly fourteen years ago there was observed in Europe
and America a beautiful comet (1861, II.), which is of interest
from several points of view. In the first place, it is one of the
212
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NUMBER OF TAILS.
comets of long period we have already mentioned it performs
its revolution about the sun in about 422 years. Moreover as
we shall see, the earth, in all probability, passed through its
tail on June 30, 1861, an event worthy of notice, if only from
the absence of any disastrous consequences to the inhabitants of
the earth. Lastly, the comet in question was remarkable at
the same date (June 30) for its beautiful fan-shaped tail, the
long divergent rays of which gave it some resemblance to the
Fig. 29. — Fan-shaped tail of the great comet of 1861, according to the observation
of June 30 and the drawing of Mr. G. Williams.
comet of 1744. The drawing which we here reproduce (fig.
29), due to Mr. G. Williams, of Liverpool, shows a striking
difference, however, in the form of the appendages of the two
comets. The divergent rays which compose the multiple tail
of the comet of 1861 are sensibly rectilinear, and emerge from
the head of the comet ; the extreme or outer rays alone, which
include an angle of 75°, are detached from the nucleus, whilst
213
THE WOULD OF COMETS.
the longer and inner rays are slightly curved, the convexity
being outwards.
Before assuming this remarkable form the great comet of
1861 was furnished with two tails of unequal length, making
an angle of about 13°. The drawings given by M. Liais, for
dates from June 19 to 28, leave no doubt upon this point.
Those which we here reproduce (fig. 30) exhibit the comet,
according to Father Secchi, as seen on
June 30 and July 2. On June 30, the
earth being exactly in the plane of the
comet's orbit, the two tails, the one long
and slender, the other shorter and of
greater width, were to all appearance
projected the one upon the other. On
July 2, the earth being then out of the
plane, they were seen as separate. Look-
ing at the drawing of Mr. Williams, which
gives the appearance of the tail on the same
day, the difference of aspect presented to
the two observers seems surprising. But
if it be true that the tail of the comet
pointed directly towards us, the diverg-
ence of the rays would be but an effect
of perspective, which would necessarily
change with great rapidity, considering
the extreme relative velocity of the move-
ments of the two bodies.
Fig. so.— The two tails of the The number therefore, as well as the
comet of 1861, according ,
to Secchi, June 30 and July form and dimensions of cometary tails,
are variable circumstances, not only as
compared one with another, but even for the same comet at dif-
ferent times ; and this variation is due to two causes ; in the
first place, to real changes taking place in the comet itself,
214
NUMBER OF TAILS.
frequently with wonderful rapidity; and, in the second place,
to the optical effects which the rapid movements of the comet
and the earth in their respective orbits necessarily produce in
the appearance of the several part's of the head, the nucleus,
and the tail.
We have still to mention, amongst comets with multiple
tails, the one which was observed in 1825 by Dunlop, in
Australia. The tail was formed of five unequal and distinct
branches. i At a distance of 1^° from the head the rays of the
several tails cross each other, and then diverge indefinitely.'
Arago, after citing this passage, mentions as a double-tailed
comet 1845, III., which ' exhibited a tail of 2.j>° long, divided
into two branches by a black line.' But, according to this
view, a great number of comets might be considered as furnished
with double tails, which in point of fact have but one, since it
often happens that the outer edges of a tail are more brilliant
than the space which separates them; and they are often of
unequal length and lustre. Thus, M. Liais considers the tails
of the great comets of 1858, 1860, and 1861 as consisting in
reality each of two tails, of which the longer and narrower is
situated in the prolongation of the radius vector, or line joining
the sun to the nucleus ; while the other, shorter but more
spread out, makes a certain angle with the former. Sometimes,
in consequence of the position of the earth with respect to the
plane of the comet's orbit, the two tails are projected the one
upon the other, and are seen as one alone : as in the case of
the comet of 1861. The question is, however, of no great
interest. The question of the multiplicity of tails is of no real
importance, except as it concerns their origin and the physical
causes which occasion their development.
215
SECTION VI.
DIFFERENT FORMS OF TAILS.
Elementary forms of tails — Rectilinear tails, divergent or convergent, in respect of
the head of the comet— Curved tails; comets of 1811 and 1769 — Whimsical
form of cornetary appendages according to ancient observations.
THE tails of comets, under whatever form they may be pre-
sented to the observer, are all, whether simple, double, or
Fig. 31 — Winnecke's comet, June 19, 1868.
multiple, easily reducible to two or three elementary forms.
In the first place, there are comets with rectilinear tails,
216
DIFFERENT FORMS OF TAILS.
that is to say, tails whose luminous rays, emerging from the
head, are projected in what appear to be right lines against the
sky. Sometimes, the tail, as in the comets of 1843 and 1769,
and that of Biela, in 1846, resembles a long ribbon of light,
nearly of the same width throughout and scarcely varying in
intensity. Sometimes it gradually narrows from the head and
tapers to a point, like the tail of Halley's comet in 1835 (see
fig. 16, page 106), Wirmecke's comet in June 1868, and
that of P. Henry in August 1873 (figs. 31 and 32). Or it
Fig. 32.— Comet of P. Henry, August 26 and 29, 1873.
may happen that the rays of a rectilinear tail may diverge
from the head and continue to diverge up to their furthest
limit, or so far as their light permits them to be seen ; of this
kind was the tail of the comet of 1686 (the aspect of which we
have given from a contemporary, J. C. Sturm), and also the tail
of the great comet of 1264. These are the forms, doubtless,
in which the ancients saw the similitude of beams, swords,
and lances. But the slightest reflection will serve to
217
THE WOIUJ) OF COMETS.
convince us that these diverse forms are apparent only, and that
the same tail may present itself under any one of these appear-
ances, according to the distance of the earth from the different
portions of thecometary appendage. As a simple consequence
of the laws of perspective the same tail may appear to be
either very short or of great length ; or in certain" cases it
may even disappear, without its real dimensions undergoing
any change.
On examining with the aid of a telescope the forms of tails
in the vicinity of the nucleus
the outline of the tail is fre-
quently observed to sweep
round and enclose the head ;
this curve bears great re-
semblance to the portion, •
near the vertex, of a parabola
or a very long ellipse, whose
focus would be the nucleus.
A case in point is supplied by
the comet of 1819, whose tail
was in the form of a cone with
nearly rectilinear boundaries ;
the great comet of 1811 like-
wise exhibited a tail whose
edges were more luminous
Fig. ss.-The comet of 1264. than the central portion, and
which was curved round the
vertex, as if to envelop the nucleus. Besides this curvature
near the nucleus the entire tail itself may be curved throughout
its length, as was the case with Donati's comet. These are
the comets like Turkish sabres, in which our ancestors of the
Middle Ages, constantly mindful of the dangers with which
the Ottoman empire menaced Christianity,
218
saw threatening
WFFETIENT FOP MS OF TAtLS.
presages of war. In all probability they belong to the class
called by the ancients the Horn, one of the kinds of comets
mentioned by Pliny. Examples of it are not unfrequently
met with in ancient drawings ; but we must not forget that
the observers of former times were not always the most exact of
draughtsmen, and that they did not hesitate on occasion to
improve upon nature according to the dictates of their fancy.
A curious instance of this mania for embellishment occurs
even in the work of Hevelius. This indefatigable and learned
philosopher, wishing to represent in his Cometographia the kind
of comet which Pliny, under the name of Xiphias, has compared
to a sword, has not failed to add the handle of the weapon. A
fac simile of this remarkable design has been given in Plate
III, fig. 9.
Cometary tails are generally curved in the same direction
throughout their whole extent ; so that one of the boundary-
lines of the tail turning its concavity to one region of the
heavens, the other boundary will turn its convexity to the
region opposite; as, for example, the comet of 1811, Donati's
comet, and many others. The two tails of the comet of 1807
were curved in opposite directions ; and a drawing of the
same comet of 1811, which we find in Chambers's Astronomy
and in the Atlas of A. Keith Johnston, represents a similar
phenomenon. A more exceptional form, and one of which we
know no other example, is mentioned by Pingre in these
terms : 4 The late M. de la Nux, at the Isle of Bourbon,
and ourselves, between Teneriffe and Cadiz, both remarked
that the tail of the comet of 1769 was doubly curved to-
wards its extremity ; it resembled the figure of an co.' But
we should bear in mind that Messier has given several
drawings of the same comet in which the tail is represented
as a rectilinear band, brighter at its edges than either at
its axis or in its interior. This last peculiarity is not un-
219
THE WORLD OF COMETS.
frequent. Nevertheless, the contrary may occur, as was
observed in the case of the cornet of 1618. 'At Rome/ says
Pingre, 'there was seen a kind of nucleus, so called by
Hevelius, in the tail of the last comet of 1618 ; it resembled
a line or a dart, which, like the pith of a tree, extended
the whole length of the tail, dividing its breadth into two parts.
Kepler and Schickard saw the same phenomenon, but it did not
then divide the breadth of the tail, it skirted along one of its
edges, which is more in conformity with what is generally
observed.'
Beyond the forms which we have just described, and which
are sufficiently regular to admit of exact definitions, the tails of
comets may assume irregular and whimsical appearances. In
the accounts extant of great and historic comets, seen with
the naked eye by observers who were often themselves as-
tronomers, we find mention made of the most singular
appearances ; but we can hardly put faith in their descriptions,
ingenuous perhaps, but certainly distorted by the superstitious
beliefs of the times. It remains for modern astronomers to
follow and to depict with scrupulous fidelity all the forms of
cometary nuclei, atmospheres, and tails, as exhibited in the
field of the telescope. The evolutions of these phenomena
are but little known, and they must be studied without pre-
conceived ideas, if we would fabricate a theory which should
be exempt from the fallacies of observers. The sole means of
discovering truth, in astronomy, as in all the natural sciences,
is to begin by collecting facts, and then, relying upon them
alone, to deduce reasons.
220
DONATI'S COMET /
as seen at Paris cm the 5th of October 1858
SECTION VII.
LENG TH OF TAILS.
Apparent and real dimensions of the largest tails on record — Formation and de-
velopment of cometary appendages ; their disappearance — Variations of length
in the tail of Halley's comet at its different apparitions — Great comet of 1858,
or comet of Donati.
SINCE we have entered upon the statistics of various cometary
elements, let us here give a few particulars respecting the real
and apparent dimensions of cometary tails. We will first
confine ourselves to the maximum dimensions under which
they have been viewed from the earth, dimensions measured in
degrees, according to the apparent extent occupied by the
train itself in the celestial vault. Passing, then, from the
apparent lengths, we will proceed to the actual measures
expressed in miles. Under the first head the scale of mag-
nitude will be found to include an enormous range, varying
from the tail of 2^°, belonging to the comet of 1851, to the
immense tail of 100°, possessed by the comet of 1264, and to
the still greater tail of the comet of 1861, which attained a
length of 118°, thus exceeding by 28° the apparent distance
between the horizon and the zenith. Nor are the differences
less considerable when we compare the true dimensions.
Whilst, for instance, the second comet of 1811 was provided
with a tail about seven millions of miles in length, the great
comets of 1811, I., 1847, I., 1687, and 1843 launched into
2L>1
THE WORLD OF COMETS.
space, in directions opposite to the sun, immense luminous
trains measuring from 109 to 199 millions of miles — more
than double the distance of the sun from the earth. Some of
these elements will be found included in the following table : —
Ptrihelion
distance.
Length of tail.
/— "• • -• -^
Apparent, in
degrees
— >
Real, in miles
Comet of 1851, I.
1-700
H
—
„ 1860, III.
0-292
15
21,700,000
„ 1825, IV.
1-241
17
—
„ 1744
0-222
24
18,600,000
„ 1811; I.
1-035
25
109,400,000
„ 1811, II.
1-582
—
6,800,000
1456
0-586
57
—
„ 1843, I.
0-005
65
198,800,000
„ 1858, VI.
0-578
64
54,600,000
„ 1689
0-019
68
—
837
0-580
79
—
„ 1680
0-006
90
149,000,000
„ 1769
0-123
97
39,800,000
1264
0-312
100
—
„ 1618, II.
0-389
104:
49,700,000
1847, I.
0-043
—
130,500,000
„ 1861, II.
0-822
118
42,200,000
The discordance between the apparent and real lengths is
striking. It is hardly necessary to point out the reason of
this discordance, as the reader is already aware that it arises
from the manner in which the tail of the comet is presented to
the observer, and depends upon the visual angle under which
a line, more or less inclined, is seen from the earth, according
to the relative positions of the earth, the plane of the comet's
orbit, and the comet itself. From the apparent length ex-
pressed in degrees, and the knowledge of the positions con-
cerned, the true length of the luminous train can be calculated
and deduced.
But the observed dimensions of the same tail are fur from
being always accordant, so that an exact estimation of the
LENGTH OF TAILS.
real length is often impossible. Jt is very difficult to dis-
tinguish the limits of a light so feeble as is that of most
cometary tails, particularly at the extremity further from the
nucleus. The clearness of the sky, the power of the instru-
ment employed, even the sight of the observer, are all so many
variable elements. On this subject Lalande has said in his
Astronomic : ' In southern countries, which enjoy a pure and
serene sky, the tails of comets are more easily discernible and
seem longer. The comet of 1759 at Paris appeared almost
destitute of tail, and it was with difficulty that a slight trace
of such an appendage was discerned, measuring one or two
degrees in length ; whilst at Montpellier M. de Ratte esti-
mated its entire length, on April 29, to be 25°, the most
luminous portion being about 10°. At the Isle of Bourbon
M. de la Nux saw it larger still, owing to the same causes
as those which permit the zodiacal light to be seen there
constantly.'
223
SECTION VIII.
FORMATION AND DEVELOPMENT OF TAILS.
Variations of length in the tail of Halley's comet at its different apparitions —
Similar phenomena exhibited by Donati's comet in 1858 — Does the maximum
development of the tail always coincide with the perihelion passage of the
comet ?
IT is now desirable to consider a phenomenon of high im-
portance as regards the physical constitution of comets, viz.,
the development and variation of their tails according to the
position which the comet occupies in its orbit ; that is to say,
according to its greater or less distance from the sun.
It has been already seen that the tails of comets frequently
are formed and developed during the period of the comet's
visibility, and generally before the perihelion passage. ' It
has been constantly observed,' says Pingre, ' that a comet
advancing to its perihelion begins to assume a tail only on its
near approach to the sun. The fine comet of 1680 had no tail
on the 14th of November, thirty-four days before its perihelion
passage. The real length of the tail increases day by day,
and the head, or rather the coma surrounding the head, seems,
on the contrary, to diminish. The tail attains its greatest
length shortly after the comet has passed its perihelion; it
then diminishes by degrees, but in such wise that at equal
distances from the perihelion the tail is longer after the peri-
helion passage than before. It has been, moreover, observed
224
FORMATION AND DEVELOPMENT OF TAILS.
that comets whose perihelion distance has much exceeded the
mean distance of the sun from the earth have net developed
tails, and that the tails of others, all else being the same, have
been more magnificent in proportion as the perihelion distances
have been less.'
Are we to consider that the laws thus enunciated by the
author of the Cometographie are general, and apply to all
known comets? No, unquestionably, as we are about to see;
nevertheless, it is certain that some relation does connect the
existence and development of the tails of comets with their
greater or less proximity to the sun.
Let us first take, for example, Halley's comet at its
apparition in 1835. When it first appeared it had the aspect
of a slightly oval nebulosity, and was thus destitute of tail.
On October 2, that is to say, six weeks before its perihelion
passage, which took place on November 16, the tail w:as
formed, and three days later it attained a length of from
four to five degrees. During the following days it continued
to increase in length, and on October 15 had attained its
maximum of 20°. On the 16th it had become reduced to
10° or 12°, on the 26th to 7°, on the 29th to 3°, and on the
5th of November to 2.2°. 'There is every reason to believe,'
says Sir John Herschel, ' that before the perihelion the tail
had entirely disappeared, as, though it continued to be ob-
served at Pulkowa up to the very day of its perihelion passage,
no mention whatever is made of any tail being then seen.'
We should add that a drawing made by Sir John Herschel
himself, on January 28, leads us to suspect an extension of a
part of the comet's atmosphere under the form of a tail ; but
on May 3, a little more than four months and a half after
the perihelion passage, the tail had completely disappeared ;
the comet had then regained its original form of a round
nebulosity.
225 Q
THE WORLD OF COMETS.
The same comet, on its apparition in 1759, was at its
perihelion on March 12. Now, on April 1, nineteen days
after its perihelion passage, the observations of Messier, made
at Paris, assign to it a feeble tail of but fifty-three minutes in
length. But, taking the observations of La Nux, which were
made at the Isle of Bourbon, under much more favourable
conditions of visibility, we find the measures of its apparent
length to be as follows: —
March 29
April 20
» 21
» 27
„ 28
May 5
3°
6° to 7<
8°
19°
25°
47°
It would be necessary to calculate the true lengths in order
Fig. 34. — Aspect of Donati's comet on December 3, 4, and 6, 1858, according to the
observations of M. Liais.
to arrive at positive conclusions respecting the development of
the tail during the two apparitions ; but it will suffice to remark
that in 1759 the tail of the comet did not attain its maximum
until long after the perihelion passage, whilst, on the con-
226
DONATI'S COMET 1858.
Formation and Development of Cemetery Appendages, from Drawings by P. G. Bond.
1. September 24, 1858. 2. September 26, 1858.
FORMATION AND DEVELOPMENT OF TAILS.
trary, in' 1835 the maximum had been attained before the
perihelion.
The great comet of Donati (1858, VI.) likewise furnishes
some interesting details on the same point. The first ap-
pearance of the tail was observed at Copenhagen and Vienna
on August 14, seventy -three days after the discovery of the
comet, and forty-six days before its perihelion passage ; it had
2930 12346 8 10 1213 161718 20 22 24 26 28 30 12 4 ' 8 10 1 |V 16 IK 20
Aug. Sept. . } Or,
Perihelion.
Fig. 35. — Variations of length in the principal tail of Donati's comet.
then an apparent length of but ten minutes. From this date
it continued to increase, and at the end of August had attained
the length of t\vo degrees. This progressive increase, which
underwent but slight fluctuations, the reader may follow either
by reference to the table given further on, which is due to
227 u 2
THE WOBLD OF COMETS.
Mr. Bond, or by: a glance at the diagram (fig. 35), in which
are represented both the apparent and real lengths of the tail.
We are here speaking only of the principal tail, which was
curved like the edge of a fan, and not of the secondary
rectilinear tails, mentioned in Section V. of this chapter. The
maximum of apparent length was attained on October 10,
eleven days after the perihelion passage; on this day the tail
measured sixty-four degrees. From this date it continued to
decrease with more rapidity than it had before increased, and
on December 3, at Rio de Janeiro, it measured only fifty-five
minutes. Three days later it disappeared, 'the comet,' says
M. Liais, * having taken a spherical form, with its nucleus
slightly eccentric, and situated in the part nearer to the sun.*
The two curves, acft, A CB, which in the figure represent
the variations in the apparent length and the real length of
the tail, exhibit a certain degree of similarity. The differences
between the two curves are due, of course, to the changes of
distance between the comet and the earth on the successive
dates of observation. Figs. 36 and 37, in which the orbits of
the comet and the earth are respectively projected, the one
upon the other, will enable the reader to determine the real
distances between the two bodies on the principal dates of
the comet's apparition, and to compare them with the varia-
tions observed in the apparent length of the comet's tail. In
order to explain completely these variations, however, we must
take into consideration all the circumstances that may affect
the visibility of the tail, and especially the brightness of the
moonlight, which would have the effect of reducing the
observed lengths in proportion to its intensity. The curve
ay/3, which in fig. 35 marks the varying intensity of the
moonlight, reaches its minimum in the nights near October 10.
This is the exact date of the maximum apparent length of
tail ; and the variations, both real and apparent, should for this
228
FORMATION AND DEVELOPMENT OF TAILS.
reason be reduced. But the law of the development of the
tail, its formation a certain time before the date of the peri-
helion passage, its increase in proportion as the comet ap-
proached the sun, its diminution, commencing a certain number
tjvns
Fig. 36. — Parabolic orbit of Dunati's comet. Projection of the earth's orbit upon the
comet's orbit. Relative positions of the two bodies.
so oct.
unot).
Fig. 37. — Projection of the orbit of Donati's comet upon the plane of the ecliptic.
Relative positions of the earth and comet.
of days after the perihelion, its disappearance, effected much
more rapidly than its development, are, in our opinion, all
incontestable facts which follow from the data which we place
229
before the reader,
to:—
THE WORLD OF COMETS.
The following is the table above referred
Length of the Tail of the Great Comet of 1858.
Date
Apparent length
in degrees
Real length
in miles
Date
Apparent length
in degrees
Eeal length
in miles -
Aug. 29
Sept. 8
, 10
2°
4°
5° 24'
14,000,000
16,000,000
Oct. 5
„ 6.
» 7
40°
50°
51°
41,000,000
45,000,000
u
, 12
6°
19,000,000
i, 8
53°
—
77
, 13
6°
20,500,000
„ 9
58°
—
77
» 16
7°
— .
„ 10
64°
54,700,000
77
» 17
8°
—
» 11
609
—
77
„ 18
5°
__
» 12
48°
—
„ 19
8°
—
„ 13
45°
39,000,000
20
6°
—
„ 14
34°
—
21
8°
_
„ 15
20°
—
22
9°
—
,» 16
10°
—
23
10°
14,900,000
,, 17
9°
—
24
10°
—
„ 18
7°
—
25
10° 30'
—
„ 19
6°
—
26
10° 30'
17,000,000
„ 21
12°
— •
27
14° 15'
—
„ 22
4°
—
28
19°
26,000,000
„ 24
4° 30'
—
29
22° 30'
—
„ 25
1°
—
30
26°
34,800,000
„ 27
4° 30'
—
Oct. 1
27°
—
„ 30
1°30'
—
„ 2
33°
37,900,000
„ 31
1°24'
— .
» 3
34°
—
Dec. 3
0° 55'
—
4
35°
—
„ 6
0°
—
The examples we have just given do not suffice to justify
the conclusion that the development of cometary tails depends
solely upon the variation of the comet's distance from the sun.
At all events, it is clear that comets show very remarkable
differences in this respect. For instance, in 1835 the tail of
Halley's comet attained its maximum length before the
perihelion, and at the date of the perihelion it had entirely
disappeared ; that of Donati's comet, on the contrary, only
•attained its maximum after the perihelion ; and two whole
months then elapsed before the comet again resumed its
original form of a round nebulosity.
To conclude our remarks upon this highly interesting but
as yet insufficiently studied subject, let us compare the comets
230
DONATI'S COMET, 1858.
Formation and Development of Cometary Appendages, from drawings by P. G. Bond.
1. October 3, 1858. 2. October 5, 1858.
FORMATION AND DEVELOPMENT OF TAILS.
whose tails we have estimated in miles with reference to their
perihelion distances. The table at page 222 will render this
comparison easy. In point of fact we see that six only — viz.
those of 1843, 1., 1680, 1847, I., 1769, 1860, III., and 1811, II.
— satisfy the condition that the lengths of their tails are greater
in proportion as the perihelion distances are smaller. The
comet of 1744 might be substituted for that of 1860, III., with-
out disturbing this relation; but the other four comets do not
conform to the rule. So that we are not justified in extending
to the comparison of comets, one with another, the law of
variation according to distance which we have seen to hold
good as regards the development of the same tail.
231
SECTION IX.
BRILLIANCY OF COMETS.
Estimations of the apparent dimensions or brilliancy of comets — Ancient comets said
to be brighter than the sun — Comets visible to the naked eye and comets seen at
noonday; great comets of 1744 and 1843.
WE will now enter into some particulars respecting the dimen-
sions of comets, their atmospheres, nuclei, and tails. In order
to form correct notions concerning this portion of our subject,
it is important to distinguish between real and apparent
dimensions. This is elementary, but it is here even more
necessary than elsewhere, because, from the very nature of
cometary orbits, the comet itself, whether periodical or non-
periodical, may be situated at the moment of its appearance
either very near to or very far from the earth ; so that on two
successive apparitions the same comet may appear of very
different aspect and dimensions, and at one time may present
itself as a very conspicuous body in the sky, at another may be
hardly visible, or perhaps not visible at all without the aid
of a telescope. We have already alluded to this point when
speaking of the difficulty of recognising the identity of a new
comet with one before observed by its external aspect ; and
we must here call attention to it again, when we are comparing
different comets in respect to their dimensions, either real or
apparent.
232
BRILLIANCY OF COMETS.
All comets whose apparitions are anterior to the sixteenth
century were visible to the naked eye ; their heads, nuclei, or
comae were, therefore, by no means insignificant; the faintest
were at least equal in brilliancy to stars of the fifth or sixth
magnitude; or if not, the extent of the surrounding nebulosity
compensated in point of visibility for the inferiority of the
nucleus. This remark applies also to all comets that have been
observed with the naked eye since the introduction of the
telescope. But, as we are aware, by the aid of instruments
comets are detected of so faint a light that they appear as
feeble nebulosities, devoid of condensation or nucleus. Many
of these last are periodical, and approach to within a moderate
distance of the earth ; so that it is not their actual remoteness
that renders it so difficult for us to see them. There is, there-
fore, amongst comets the same diversity of dimensions and
brilliancy as amongst the stars.
Certain comets have been of enormous dimensions and of
great brilliancy. Ancient traditions testify to this intensity,
but We must not rely too implicitly upon accounts derived
from these sources, for they contain evident exaggerations. As
such, for instance, we must regard the comet of B.C. 183,
4 which was more brilliant than the sun, and was seen by day-
light in Pisces ' ; and the comet mentioned by Seneca, which
appeared B.C. 146. ' After the death of Demetrius, King of
Syria, the father of Demetrius and Antiochus, there appeared
shortly before the Achasan war a comet as large as the sun.
At first it was like a disc of fiery red, and its light dissipated
the darkness of the night. Imperceptibly it decreased in size,
its light became dim, and it totally disappeared.' Again, the
comet which appeared B.C. 136, at the birth of Mithridates,
and remained visible for seventy days, seems to have been
somewhat magnified by the imagination of observers and
historians. ' The heavens appeared on fire ; the comet
233
THE WORLD OF COMETS.
occupied the fourth part of the sky, and its light exceeded that
of the sun.1
In the Cometographie of Pingre we find the following
description of comets which were remarkable either for their
dimensions or the brilliancy of their light:—
1 1006. Haly ben Rodoan being young, a comet was seen
in the 15th degree of Scorpio; the head was three times as
large as Venus ; it gave as much light as a quarter of the moon
would have, given.'
' 1106. A great and beautiful comet. On the 4th, or,
according to others, on the 5th of February, a star was first
seen distant only a foot and a half from the sun ; it was there
beheld from the third to the ninth hour of the day. Some
authors have given to this star the name of comet.
' 1208. In this year there appeared a comet. For a fort-
night, after sunset, a star was seen of such brilliancy that it
produced a great light, not unlike a fire. The Jews regarded
it as a sign of the coming of the Messiah.
' 1402. A very large and very brilliant comet; no one
remembers to have seen such a prodigy. (This is believed to
be a prior apparition of the comet of 1532 and K 61.). . . It
increased day by day in size and brilliancy as it drew near the
sun. On Palm Sunday, the 19th of March, and the two
following days, it increased prodigiously; on Sunday, its tail
was twenty-five fathoms long ;* on Monday, fifty, and even
* In ancient chronicles we frequently find in descriptions of the apparent
dimensions of celestial bodies, and more especially the tails of comets, expressions
similar to those here mentioned ; that is to say, lengths expressed in ordinary
measures — two or three feet, or twenty or a hundred fathoms, &c. It is plainly
impossible to give any rational meaning to such statements. Nor have similar
expressions for the same kind of estimates entirely passed out of use even at the
present day. A person who sees a bolide will say, for instance, that its size was
that of an orange, and that it had a train two yards long. He does not under-
stand that this manner of measuring the apparent dimensions of objects, whose
real distance is unknown, is altogether indeterminate, and that, although it may
234
BRILLIANCY OF COMETS.
one hundred ; on Tuesday, more than two hundred. It then
ceased to be visible at night, but during the eight following
days it was seen in the daytime close to the sun, which it pre-
ceded. Its tail was not more than one or two fathoms long ;
it was so bright that the light of the sun did not prevent it
being seen at noon-day? In 1532, if the identification be cor-
rect, the same comet exhibited a degree of brilliancy equal
to three times that of Jupiter.
Thus, there have been several comets sufficiently brilliant
for their light to have been compared to that of the sun.
Three of these were visible during the day. The great comet
of 1500, known under the names of Asta, and // Signor Astone,
was likewise seen in presence of the sun. ' Some voyagers
sailing from Brazil to the Cape of Good Hope saw it on the
12th of May; it appeared on the Arabian side of the vessel.
Its rays were very long. It was thus continually observed day
and night for eight or ten days.'
That comets have been brilliant enough for their light to
penetrate the sunlit heavens is put beyond a doubt *by the
observations made by the celebrated Tycho of the comet of
have a precise meaning for him at the moment when he sees the object, it does
not follow that at another time his estimate would not be quite different. And,
in any case, such estimates, made by different observers, are not comparable one
with another. The only proper mode of expressing celestial distances is in
degrees and minutes, and the observer, not provided with an instrument, and not
accustomed to making such estimations, will find it useful to remember that the
diameters of the sun and the moon are pretty nearly equal, and that the diameter
of each is about half a degree. By comparison with either of these luminaries
it is easy to make a good estimate of a celestial distance. We may also — and this
is a good plan, if it be a starlight night — compare the length to be measured
with the distance between two well-known stars in any of the constellations,
such as the Great or Little Bear, Orion, Pegasus, Cassiopeia, &c. We especially
dwell upon this matter, because more than once we have had occasion to deplore
the. method of measuring celestial dimensions in feet, yards, &c., a method
of measuring absolutely without meaning, and which may render valueless an
observation which might otherwise be important.
235
THE WORLD OF COMETS.
1577, and those of contemporary astronomers of the great
comet of 1843. l On the 13th of November, 1577, whilst the
sun was still above the horizon, this new star (the comet)
caught the attention of Tycho Brahe. He estimated the
diameter of its head at seven minutes.'
With respect to the comet of 1843 the following details,
which we borrow from Arago, leave no doubt of its visibility in
full sunlight : l The comet, first perceived by the spectators
in broad daylight, and thought to be a meteor, was, at the hour
of noon, according to an observation made by M. Amici, 1° 23'
east of the centre of the sun. M. Amici says only of the body
that it was famous towards the east. At Parma the observers
aver that whilst stationed behind a wall screening the sun
from view they distinctly saw a tail of from four to five
degrees in length. In Mexico, on the same day (the 28th
of February), at eleven o'clock in the morning, according to
the Diario del Gobierno, " the comet was visible to the naked
eye, near to the sun, like a star of the first magnitude, with the
first development of a tail directed towards the south." Mr.
Bowring, at the mines of Guadaloupe y Calvo (Mexico), saw
the comet on the 28th of February, from nine o'clock in the
morning until sunset. At Portland (U.S.) the comet was seen
with the naked eye in open daylight, to the east of the sun, by
Mr. Clarke. Sir John Herschel makes mention of an obser-
vation made by one of the passengers on board the Owen
Glendower, then off the Cape. " The comet was seen as a
short dagger-like object, close to the sun, a little before sun-
set." According to Mr. Clarke, " the nucleus and also certain
parts of the tail were as clearly defined as the moon on a
clear day." ' *
[* Mr.E.C. Otte, in his translation of Humboldt's ' Cosmos' (vol. i., p. 86),
states that at New Bedford, Massachusetts, U.S., on February 28, 1843, he
distinctly saw the comet between one and two in the afternoon. The sky at the
time was intensely blue, and the sun shining with a dazzling brightness un-
known in European climates. — ED.]
236
BRILLIANCY OF COMETS.
A century before, in 1743, a comet was observed in Europe
— Cheseaux's comet, which we have several times mentioned —
which surpassed in brilliancy stars of the first magnitude. On
January 9, 1744, the head of the comet was equal to a
star of the second magnitude, and its diameter fifteen days
after amounted to ten seconds; on January 26 it was equal
to a star of the first magnitude ; on February 1 it was brighter
than Sirius ; and finally, during the last few days of this
month and the commencement of March, it became so bright
that it was visible by daylight in presence of the sun. But a
remarkable circumstance related by Cheseaux is this : l From
the 13th of December to the 29th of February (on the follow-
ing day, the 1st of March, the comet passed its perihelion) the
atmosphere of the comet continued to diminish in size,' as if
the augmented brilliancy of the head was produced by the dis-
appearance of the nebulosity surrounding the nucleus, or by a
condensation of the nebulous atmosphere.
237
SECTION X.
DIMENSIONS OF NUCLEI AND TAILS.
Real dimensions of the nuclei and atmospheres of various comets— Uncertainty of
these elements; variations of the nucleus of L'oiiati's comet— Observations of
Hevelius upon the variations of the comet of 1652— Do cometary nebulosities
diminish in size when their distance from the sun decreases ? — Encke's comet
considered in regard to this question at its apparitions in 1828 and 1838.
THE observations that we have just recorded give an idea of
the brightness of cometary light, and the intensity to which
that brightness may attain ; but they afford no certain indica-
tion concerning the dimensions of cometary nuclei or atmo-
spheres. Upon this point we are about to give the result of
a few measurements ; but these measurements, it must be
understood, are not so exact as those of the bodies of the solar
system, the planets, the moon, and sun. The uncertainty we
speak of does not arise from the difficulties experienced in
the determination of the measures themselves, although they
contribute to it, cometary nuclei being often as deficient in a
clear and well-defined outline as the nebulosities ; but what
more especially prevents us from regarding the numbers we
now give as constant, and therefore characteristic elements of
the comets to which they belong, is the continual variation to
which the different parts of the head are subject during the
time of the comet's apparition.
238
DIMENSIONS OF NUCLEI AND TAILS.
The following two tables contain the values obtained for
the dimensions of various cometary nuclei and atmospheres,
arranged in order of magnitude : —
Diameters of Cometary Nuclei.
Comet of 1798, I. .
„ 1805
„ 1799, I. .
1811, I. .
1807
„ 1811, II. .
181VI. .
1847, I. .
1780, I. .
1843, I. .
1815 ..
„ 1858, VI.
1769
Miles.
28
30
385
429
550
2,700
3,300
3,500
4,200
5,000
5,300
5,600
28,000
Diameters of Cometary Atmospheres.
Comet of 1799, I.
„ 1807
1847, V.
1847, I.
1849, II.
1843, I.
„ Brorsen,1846
„ Lexell, 1770
„ 1846, I.
„ Encke, 1828, .
„ 1780, I.
„ Halley, 1835
1811, I.
Miles.
1,200
1,900
17,900
25,400
50,700
94,500
129,000
203,000
241,000
264,000
267,000
354,000
1,120,000
On comparing these tables we find that the six comets,
1799, I., 1811,1., 1807, 1847, I., 1780, I., and 1843, I , whose
nuclei and atmospheres have been both measured, do not
occupy the same relative positions in each. This is strikingly
shown in the case of the great comet of 1811. whose some-
239
THE WORLD OF COMETS.
what small nucleus was surrounded by an immense nebulosity,
as is evident from the large number in the last line of the
second table. The volume of the nucleus was only equal
to the 6,300th part of the volume of the earth, whilst that
of the coma was 2,800,000 times greater than the volume of
the earth; that is to say, more than double the volume of the
sun.
In order to justify the remarks at the beginning of this
section, let us take for example the beautiful comet of Donati,
whose physical elements have been so carefully studied by
Bond. The diameter of 5,600 miles, given in the table,
has reference to the nucleus on July 19. On August 30
this diameter was reduced by one-sixth, and measured no
more than 4,660 miles. It continued to decrease until October
5, on which day it did not exceed 400 miles, less than -jLth
of its diameter on July 19. The next day it attained 800 miles,
having doubled its dimensions between one day and the next,
the volume of the nucleus having thus been increased in the
proportion of 1 to 8. Finally, on October 8 the diameter
attained a new maximum of 1,120 miles, and on the 10th was
reduced again by one -half, viz. to 630 miles. We do not
now enter into the significance of these rapid variations, of
which we shall have to speak hereafter, when treating of the
physical constitution of cornetary nuclei. It will then be
seen that these variations appear to be connected with the
changes of distance between the nucleus of the comet and
the sun.
Hevelius, in the sixth book of his Cometographia, describes
the physical aspect of the comet of 1652, the magnitude of the
head and tail, together with the brilliancy and the colour of
their light. He observes that, the apparent dimensions of the
comet having diminished clay by day, this diminution was the
natural result of the continually increasing distance between
240
DIMENSIONS OF NUCLEI AND TAILS.
the comet and the earth, but that in reality the absolute size
of the comet was increasing day by day. This observation,
the value of which Pingre' denies, because he does not believe
that Hevelius could have measured with sufficient accuracy
the dimensions of the comet or calculated its distances from
the earth, has since been generalized, and several astronomers,
including Newton, have remarked that the diameters of come-
tary nebulosities increase in proportion as the comet becomes
more and more distant from the sun. Arago observes that
the comets of 1618, II., and 1807, manifestly exhibited this
phenomenon. It has, however, been better exemplified by
Encke's comet in its two apparitions of 1828 and 1838. The
following table shows these remarkable variations: —
Real Diameters of Encke's Comet in 1828.
Days
Distance from the sun
Diameter in miles
October 28
1-46
323,000
November 7
1-32
263,000
30
0-97
122,000
December 7
0-85
82,000
14
0-73
45,000
» 24
0-54
12,000
The diminution of the diameters is much more rapid than
that of the distances from the sun ; the six distances decrease,
in fact, in the proportion of the numbers 100, 90, 65, 58, 50,
and 36, whilst the corresponding diameters are to each other
as the numbers 100, 81, 38, 25, 14, and 4; the distances being
at length reduced to a third nearly, whilst the diameter is
twenty-six times less; and if we pass from the diameter to
the volume of the nebulosity, it will be found that between
October 28 and December 24 the volume was reduced to the
17,600th part of its original value.
We now proceed to the variations exhibited by the same
comet in 1838,.the elements of which are as follows : —
241 R
THE WORLD OF COMETS.
Heal Diameters of Encke's Comet in 1838.
Days
Distance from the sun
Diameter in miles
October 9
1-42
278,000
25
1-19
119,000
??
November 6
13
1-00
0-88
80,000
75,000
?7
16
0-83
62,000
7?
„ 20
0-76
55,000
23
0-71
37,000
77
24
0-69
30,000
77
December 12
0-39
6,500
» 14
0-36
5,500
„ 16
0-35
4,200
„ 17
0-34
3,000 '
Fig. 38. — Encke's comet, according to the observations of Schwabe. 1. October 19, 1838 ;
2. November 5 ; 3. November 10 ; 4. November 12.
From October 9 to December 17 the distance of the comet
from the sun was reduced in the proportion of four to one,
while the real diameter of the nebulosity was reduced to the
242
DIMENSIONS OF NUCLEI AND TAILS.
93rd part of its value; and its volume — supposing the comet
to be spherical, or, at all events, only changing its size, not its
shape — was reduced to the 813,000th part of the original volume.
It appears even certain that on this second apparition the law
of decrease of the diameter as compared with the diminution of
the distance from the sun followed a still more rapid law of
variation. Moreover, it should be remarked that, at equal
distances, the comet in 1838 was somewhat less in volume
than ten years before. We limit ourselves at present to the
statement of the fact, as we shall give further on the
explanations offered, and the difficulties which it presents in
respect to the physical constitution of comets. As some con-
nexion has been thought to exist between the changes of
volume in cometary nebulosities, and the development and
formation of tails, we may here remark that Encke's comet is
a nebulosity of variable form — sometimes globular, sometimes
oval, or more or less irregular (figs. 17, 22, 23, and 38), and
that at no time has it ever exhibited a tail.
243 R 2
CHAPTER VIII.
PHYSICAL TRANSFORMATIONS OF COMETS.
SECTION I.
AIGRETTES— LUMINOUS SECTORS — NUCLEAL EMISSIONS.
Predominance of atmosphere in comets — Luminous sectors ; emission of vaporous
envelopes from the nucleus in the comets of 1835, 1858, 1860, and 1861 — Forma-
tion t)f envelopes in Donati's comet ; progressive diminution of the velocity of
expansion in emissions from the nucleus.
THE planets, as seen through a telescope, are bodies of regular
form and definite dimensions, probably invariable, so far as
we can judge from observations made in the short period of
two centuries and a half that has elapsed since telescopes have
been invented. A globular mass, solid or liquid, surrounded on
all sides by a light and comparatively thin aeriform envelope,
is perhaps, from a physical point of view, the simplest
description of a planet. The comparative stability is due,
on the one hand, to the preponderance of the central globe,
where general phenomena are modified only at long intervals ;
and on the other to the trifling depth of the atmosphere,
the portion of the planet the most subject to variation and
internal change.
In comets, we have seen, this relation is reversed, and
the atmosphere or nebulous envelope constitutes the entire
body, or at all events greatly preponderates. At the utmost
we can only conjecture that in some comets the nucleus is solid
or liquid. Certainly its volume is generally but a very insig-
nificant portion of the entire nebulosity, even if we except
247
THE WORLD OF COMETS.
the tail.* A comet which in one part of its orbit seems to be
reduced to a simple nebulosity will gradually exhibit a lumi-
nous condensation and then a nucleus. This nucleus either
increases or decreases in volume and brightness. Nothing
appears stable in the constitution of these remarkable bodies ;
variability of aspect is one of their most distinctive features.
We have already seen that the nucleus and the atmosphere of
a comet undergo considerable changes in the course of the
same apparition; the enormous appendages of certain comets
are generated, take form, and develop only to dimmish and
then disappear. It remains now to study the internal changes,
changes that require for their observation instruments of the
greatest power, to examine if there is not some connexion
between the phenomenon of tails and the movements of the
coma and the nucleus, and whether they are not connected
with some external influence, such as the solar heat or other
natural action. All comets which describe orbits of con-
siderable eccentricity must in the course of a single revo-
lution be exposed to enormous differences of temperature ;
and the extreme variations of heat and cold to which they are
subjected between their perihelia and aphelia cannot fail to
create in these masses of vapour, gas, or particles disseminated
over enormous volumes — whichever they may be — movements
of contraction and dilatation, perhaps even of chemical action,
of which upon our globe we can have no idea. The pheno-
mena of solar spots and protuberances are alone comparable
with these rapid and singular transformations.
But let us now proceed to the facts which justify these
conjectures and invest them with a high degree of probability.
* The tables on page 238 show that the volumes of the nuclei, in the
comets of 1799 and 1807, only amount to T-^ of the volumes of the nebulosities.
This proportion decreases to ,^ in the comet of 1843, and to ^nnnnrouoooo in
the great comet of 1811.
248
Warren. De La, Rue. del .
'y
THE GREAT COMET OF 1861
AS SEEN BY WA R R E N DE LA RUE. DCL.F R.S
WITH HIS NEWTONIAN EQUATOREAL
OF 13 INCHES APERTURE
AIGRETTES— LUMINOUS SECTORS— NUCLEAL EMISSIONS.
The continual changes of which the heads of a certain
number of comets are the seat were first put on record by
Heinsius, when observing at St. Petersburg the great comet of
1744 (Che'seaux's comet). ' On the 5th day of January,' says
Arago, 'Heinsius saw nothing extraordinary about the comet;
but on the 25th he discovered aluminous aigretle,m the form of
a triangle, the apex of which was at the nucleus, whilst the open-
ing was directed towards the sun. The lateral edges of the
aigrette were curved, as though driven in from outside by the
action of the sun. On the 2nd of February these same edges,
still more curved, formed the two sides of the commencement
of a tail, which became more distinct on the following day.'
No other observations of the same kind were made until
the return of Halley's famous comet in 1835, when the for-
mation of luminous sectors, which seemed to spring from the
nucleus towards the sun, the variations of their position,
number, and brilliancy, and other curious and instructive
phenomena, were observed in various parts of Europe : at the
Observatory of Paris, by M. F. Arago; at Dessau and Konigs-
berg, by Schwabe and Bessel ; at Markree, Ireland, by Mr.
Cooper ; at Florence, by M. Arnici. From October 7 to
November 10 the head of the comet presented a succession
of appearances of which we subjoin a few examples. (See
Plate VI., in which the variations of the comet's atmosphere
are represented, according to the observations of Sir John
Herschel at the Cape of Good Hope ; and fig. 39, in which
these appearances are given according to Schwabe.) By the
study and interpretation of these phenomena Bessel, the illus-
trious astronomer of Konigsberg, directed the attention of
savants and observers to this hitherto much-neglected branch
of cometary astronomy ; and Arago likewise contributed to the
same object by various popular notices in the Annuaire du
Bureau des Longitudes. Bessel has particularly dwelt upon one
249
THE WORLD OF COMETS,
fact of high importance: he remarked that the luminous coma,
sector, or aigrette emanated from the nucleus, and was at first
emitted in the direction of the radius vector; it then deviated
gradually, and by a marked amount, from its first direction,
and finally returned to its original position and deviated again
in the opposite direction. He was thus led to infer the exist-
ence of a movement of rotation, or rather oscillation of the
head and nucleus in the plane of the orbit. It is this oscillation
Fig. 39. — Luminous sectors and aigrettes of Halley's comet, according to Schwabe. (1)
October 7, 1835; (2) October 11; (3) October 15; (4) October 21; (5) October 22;
(6) October 23.
which has given rise to the hypothesis, remarkable in all
respects, of the existence of a polar force, having its focus of
action in the sun, and which causes cometary bodies to oscillate
just as a bar magnet causes a magnetic needle to vibrate.
Further on we shall devote a section (Chap. XL, Sec. V.)
to the exposition of Bessel's theory.
250
AIGRETTES— LUMINOUS SECTORS— NUCLEAL EMISSIONS.
Four other comets have presented analogous phenomena,
but with differences that we shall proceed to mention. These
are the comet of DonatiJ 858, those of 1860, III., 1861, II., and
lastly the comet of 1862, II., concerning which we shall enter
into some details. These details will explain the formation
and succession of the luminous aigrettes, or sectors, the
nebulous envelopes to which they give rise, and lastly the
formation of the tail, which the cometary matter that has
Fig. 40. — Formation of luminous sectors and Fig. 41. — Comet of 1860, III. June 27, ac-
envelopes. Donati's comet, Sept. 8, 1858. cording to Bond. Aiyrettes and envelopes.
thus left the nucleus appears to originate under the influence
of a kind of repulsion, the cause of which we shall have
later on to consider.
In Donati's comet the jets of luminous matter liberated
from the nucleus in the form of luminous sectors, disposed like
a fan, produced around the head successive envelopes, which as
they receded from the nucleus diminished in brightness and
became uniformly blended. This kind of compression was
251
THE WOULD OF COMETS.
regarded by Bond as the result of progressive diminution ill the
velocity of expansion of each envelope. Seven successive
envelopes, rising above the nucleus, were formed in periods
varying from 4 days 16 hours to 7 days 8 hours. Each succes-
sive envelope as it arose remained as if retained by the nucleus
for a certain time, until, in virtue of some acquired property, it
drifted back and contributed to form the two main divisions of
the tail. The sectors always appeared in the same direction,
Fig. 42. — Luminous envelopes of Donati's
comet. September 30, 1858.
Fig. 43. — The same comet. October 2.
From a drawing by Bond.
facing the sun, so that we may conclude that neither the
nucleus nor the coma was endowed with a movement of
sensible rotation ; thus no oscillation of the kind observed by
Bessel was manifested in the head of Donati's comet, except
the motion necessitated by the constant direction of the lumi-
nous sectors towards the sun. Even this absence of rotation,
according to Bond, implies the action of a polar force, ema-
252
AIGRETTES— LUMINOUS SECTORS— NUCLEAL EMISSIONS.
nating from the sun, and maintaining the axis of the nucleus
in the direction of the focus of movement.
The comets of 1860 and 1861 were also the seats of nu-
cleal emissions in a permanent direction, the first for a fort-
night, the second for a month. Eleven successive envelopes
Fig. 44.— Formation of the luminous envelopes Fig. 45. — The san?e. October 8. Both
in Donati's comet. October 6. from drawings by Bond.
were emitted from the nucleus of the comet of 1861, at
regular intervals of two days. The development and final
dispersion were thus accomplished with much greater rapidity
than in the case of Donati's comet.
253
SECTION II.
OSCILLATIONS OF LUMINOUS SECTORS: COMET OF 1862.
M. Chacornac's observations upon the comet of 1862— Formation of luminous sectors
emanating from the nucleus — Oscillation of aigrettes, and flowing back of the
nucleal matter.
WE are now about to give our attention to the evolutions of the
luminous sectors of the great comet of 1862, which, on the
contrary, presented oscillations analogous to those exhibited
by the aigrettes of Halley's comet. We shall follow the de-
velopment of these phenomena by means of the observations
of the late M. Chacornac.
On August 10, 1862, M. Chacornac detected in the head
of the comet the presence of a luminous aigrette, a brilliant
sector directed towards the sun. This sector, which at three
o'clock in the morning included an angle of 46°, had, by two
o'clock on the following day, opened ' like the corolla of a
convolvulus, and included 65°. On the 10th the nucleus pre-
sented the appearance of a rocket, having a diameter much
more extended in the direction of the radius vector than at
right angles to it.' It is worthy of remark that the contrary
was the case with the nuclei of the comets of 1858 and 1861,
which were flattened in the direction of the radius vector. On
the llth the two diameters were nearly equal. New sectors
disengaged themselves successively from the nucleus, and on
August 26 M. Chacornac determined that between the 10th
254
OSCILLATIONS OF LUMINOUS SECTORS: COMET OF 1862.
and the 26th they had succeeded each other to the number of
thirteen.
Having carefully followed throughout this interval (with
the exception of three nights, when the sky was cloudy) the
formation of these successive sectors, M. Chacornac has given
in the following terms a brief description of the phenomena
which he observed: —
' The nucleus of the comet emits periodically, in the direc-
tion of the sun, a gaseous jet from which particles of cometary
matter escape like steam escaping from a piston. This jet pre-
serves for a certain time a rectilinear form, as if a force of con-
siderable projecting power, residing in the nucleus, threw off
particles in that direction ; then it becomes inflected and takes
the form of a slightly arched cone. At this same moment the
cometary matter, accumulating at the extremity of the jet
nearer to the sun, forms a kind of cloud, the rounded outline
of which would appear to indicate that at this distance from
the nucleus the force of projection has been overcome by some
resistance opposed to it ; the cloud returns on both sides, like
a puff of smoke driven back by the wind ; and, opening out into
a level sheet, flows away in the direction of the tail.'
'By degrees the vaporous cone, the axis and vertex of
which have continued to appear the most luminous portions,
assume a diffused and nebulous appearance, as if veiled by an
accession of thick atmosphere ; the brightness of the centre
diminishes, that of the sides increases, and the cone enlarges.
The diffused appearance continuing to increase, the gaseous
jet loses its form, the light of the axis disappears, and every-
thing seems to indicate that the nucleal emission has ceased
in this direction. The nucleus appears round and brilliant.
At this time, at an angle with the radius vector of about
30 degrees towards the east, appear the first traces of a new
jet destined to succeed the former ; and, in proportion as these
255
THE WORLD OF COMETS.
traces become more apparent, the vaporous jet, originally
directed to the sun, continues to enlarge and to curve more
and more, until at last, having gradually changed its form and
become reduced by imperceptible degrees to a misty haze, it
hardly retains a trace of its primitive shape and direction. In
this state the hemispherical envelope surrounding the aigrette
is more brilliant and better defined in the portion corre-
sponding to the jet in process of dispersion than elsewhere.
'During the dispersion of the jet directed to the sun the
new jet has been gradually progressing like the first; that is to
say, the nucleus has been lengthening by degrees into the form
of a cone and disengaging particles from every part of its
surface, which, thrown off in the direction of the radius vector,
have been actively forming the new jet, which is destined
sixteen hours later to pass through the same phases as its pre-
decessor. This new jet exhibits the same changes as the
previous one, with this exception, however, that it feeds the
eastern portion of the hemispherical envelope, and the other
branch of the tail. From the observations which I was
enabled to make, it seems that these nucleal emissions have
been taking place alternately since August 9, each ray or jet
directed towards the sun, or nearly so, being succeeded by
another ray or jet inclined to the preceding, so that the
number of vaporous jets emitted by the nucleus, from that
date up to ten o'clock on the night of August 26, would
amount to thirteen. Since the date of the comet's perihelion
passage the jet which corresponded very nearly to the direction of
the radius vector has gradually inclined to the west, so that
the other jet, turned towards the east, is now directed to the
sun.'
On comparing these highly interesting phenomena with those
previously described it is impossible to avoid being struck, not
only by the degree of similarity they present to the phenomena
256
OSCILLATIONS OF LUMINOUS SECTORS: COMET OF 1862.
which attracted the attention of Bessel in Halley's comet, hut
by the differences between these same phenomena and those
which were observed in the great comet of 1858. It does not
appear that any trace of oscillation was manifested in the
luminous sectors of the latter, whilst the development of con-
centric envelopes was, on the contrary, the distinctive feature.
On the other hand, in the comets of 1835 and 1862 the
oscillatory motion, more or less rapid, chiefly characterised
the jets of vapour and luminous matter emitted by the
nucleus. These differences and analogies it will be necessary
to bear in mind, when we endeavour to trace to a unique
physical cause the transformations that are continually occur-
ring within the atmospheres of comets, especially in the vicinity
of the perihelia.
267
SECTION III.
DUPLICATION OF BIELA'S COMET.
First signs of the doubling of Biela's comet, in the mouth of January 1840 — Observa-
tions of the twin comets in America and Europe — Gradual separation and
approach of the fragments — The two comets return and are observed in 1852 ;
their distances found to have increased— Elements of the orbits of the two
comets.
WE now come to transformations still more singular in the
outward appearance of cometary nebulosities, and more radical
in their nature.
The second return of Biela's comet (period 6f years) since
the epoch of its discovery as a periodical comet in 1826, or
the eleventh of its returns since it was first observed in 1772,
was marked by a memorable event, viz. its duplication and
division into two distinct and separate comets. We here
subjoin a few details on the subject of this event.
On December 21, 1845, the comet was observed by Encke
at Berlin; on the 25th of the same month it was seen by M.
Valz at Marseilles. Neither of these two astronomers per-
ceived the slightest trace of separation. On the 19th, however,
Mr. Hind remarked towards the north of the nucleus what
appeared to be a kind of protuberance : was this a premonitory
bign of the doubling of the comet ? However this may be,
it appears certain that the comet was first seen to be double
on January 13, 1846, at Washington. In Europe the existence
2o3
DUPLICATION OF BIELA'S COMET.
of two separate nuclei was observed by Professor Challis,* at
Cambridge, on January 15; and by M. Valz at Marseilles, and
Encke at Berlin, on the 27th, only fifteen days before the
perihelion passage of the comet.
[* The doubling of Biela's comet was so remarkable an event in the history
of cometary astronomy, that I think Professor Challis's own account of what
he saw will be found interesting to the reader. He announced the extra-
ordinary appearance of the comet to the President of the Koyal Astronomical
Society as follows : —
' On the evening of January 15, when I first sat down to observe it, I said to
my assistant, " I see two comets." However, on altering the focus of the eye-
glass and letting in a little illumination the smaller of the two comets appeared
to resolve itself into a minute star, with some haze about it. I observed the
comet that evening but a short time, being in a hurry to proceed to observations
of the new planet. On first catching sight of it on this evening (Jan. 23) I
again saw two comets. Clouds immediately after obscured the comet for half an
hour. On resuming my observations I suspected at first sight that both comets
had moved. This suspicion was afterwards confirmed : the two comets have
moved in equal degree, retaining their relative positions. I compared both with
Piazzi, Oh 120, and the motion of each in 50m was about 7s in R.A. and 10'' in
N.P.D. What can be the meaning of this ? Are they two independent comets ?
or is it a binary comet ? or does my glass tell a false story ? I incline to the
opinion that this is a binary or double comet, on account of my suspicion on
Jan. 15. But I never heard of such a thing. Kepler supposed that a certain
comet separated into two, and for this Pingre said of him, " Quandoque bonus
dormitat Homerus" I am anxious to know whether other observers have seen
the same thing. In the meanwhile, I thought, with the evidence I have, I had
better not delay givii.g you this information.'
In a subsequent letter Professor Challis says : ' There are certainly two
comets. The north preceding is less bright and of less apparent diameter than
the other, and, as seen in the Northumberland telescope, has a minute stellar
nucleus. I compared the two comets independently with A.S.C. [Astronomical
Society's Catalogue] 51 on the evenings of January 23 and 24, and obtained the
following places . . .
' The greater apparent distance between the comets on January 24 is partly
accounted for by their approaching the earth. I saw the comets on January 25,
but took no observation. The relative positions were apparently unchanged.
* I think it can scarcely be doubted, from the above observations, that the
two comets are not only apparent!}7 but really near each other, and that they are
physically connected. When I first saw the smaller, on January 15, it was faint,
and might easily have been overlooked. Now it is a very conspicuous object, and
a telescope of moderate power will readily exhibit the most singular celestial
phenomenon that has occurred for many years — a double comet.'
259 s 2
THE WOELD OF COMETS.
1 On the 18th and 20th of January,' says M. Valz, 'there was
nothing remarkable in the appearance of the comet; but the
central luminous condensation seemed to be more intense than
on preceding apparitions. Cloudy weather did not permit me
to see the comet again until the 27th. I was then struck with
amazement to find two nebulosities, separated by an interval of
2', instead of one nebulosity alone . . . Yesterday, on the 29th,
in spite of clouds, I again observed the double head ; the
secondary head is much fainter than the other.' Each head
was followed by a short tail, whose direction was perpendicular
to the line joining the centres of the nuclei. The two nuclei
were moving with the same velocity and in the same direction.
On January 31 Mr. Hind verified the rapid separation of the
nuclei. Less than a month later the distance between the
twin comets had tripled, and the aspect of each varied from
day to day. Sometimes the one nucleus would excel in bright-
ness, sometimes the other, so that it was difficult to say which
was the original comet and which the secondary.
Fig. 46 shows the aspects and relative positions of the
nuclei and their tails on February 21, according to a drawing
of the Russian astronomer, Otto Struve. At this time there
was no apparent connexion, no material communication be-
tween the two bodies. ' The part of the heavens separating them
was,' as Humboldt observes, ' remarkably free from all nebu-
losity, as seen at Pulkowa. Now, some days later, Lieutenant
Maury observed at Washington, with a telescope furnished
with a Munich object-glass of 9 inches diameter, rays sent
out by the old comet towards the new, so that for some time a
kind of bridge extended from the one to the other. On the 24th
At Konigsberg, M. Wichmann observed the comet on the 14th, and saw
nothing of the companion. There was, however, some vapour in the air. On
January 15, the air being purer and the moon not risen, he saw the companion
comet immediately with a power of 45. — Monthly Notices of the Royal Astro-
nomical Society, vol. vii. pp. 73-75 (March 13, 1846).— ED.]
DUPLICATION OF BIELA'S COMET.
of March the little comet, insensibly diminishing in brightness,
was hardly recognisable. The larger one, however, continued
visible until about the 1 6th or 20th of April, when it also dis-
appeared.' ('Cosmos,' vol. iii.)
The increase of the apparent interval between the two
nuclei proved no actual increase of distance between the two
fragments of the comet, since they were approaching the
earth during the time of observation ; but the calculation of
the true distances was performed by M. Laugier and subse-
quently by M. Plantamour and M. D' Arrest; and it results from
Fig. 4.6. — Biela's comet after the duplication on February 21, 1846. According to Struve.
the following table, due to the last-named astronomer, that the
two cornets continued to separate till February 13, and after
that date gradually approached one another: —
Distance between the two nuclei, 1846 — January 14, 177,000 miles.
„ 24, 186,000 „
February 3, 191,000 „
„ 12, 193,000 „
„ 23, 191,000 „
March 5, 190,000 „
„ 15, 180,000 „
„ 25, 172,000 „
The variations of brilliancy and size presented by the two
201
THE WORLD OF COMETS.
comets were not less remarkable than the variations of the
distance between them. Both had nuclei, both had short tails,
parallel in direction and nearly perpendicular to the line of junc-
tion. ' At its first observation, on January 13, the new comet
was extremely small and faint in comparison with the old, but
the difference both in point of light and apparent magnitude
continually diminished. On the 10th of February they were
nearly equal, although the day before the moonlight had effaced
the new one, leaving the other bright enough to be well ob-
served. On the 14th and 16th, however, the new comet had
gained a decided superiority over the old, presenting at the
same time a sharp and starlike nucleus, compared by Lieut.
Maury to a diamond-spark. But this state of things was not to
continue. Already, on the 18th, the old comet had regained its
superiority, being nearly twice as bright as its companion, and
offering an unusually bright and starlike nucleus. From this
period the new companion began to fade away, but continued
visible up to the 15th of March. On the 24th the comet ap-
peared again single, and on the 22nd of April both had dis-
appeared.' (Herschel, 'Outlines of Astronomy.')
The luminous communication, mentioned above, which
Maury observed between the two bodies is also worthy of
attention. Besides the tails of the comets Maury saw a fine
luminous arc, which extended from one nucleus to the other
like the arch of a bridge. This was when the new comet was
at its maximum brilliancy: and when the old comet had re-
gained its superiority, it threw out new rays, which gave it the
appearance of a comet with three tails, making angles of 120°
with one another, and one of which joined the two comets.
These curious phenomena raise questions of the highest
interest. What cause determined the separation of Biela's
comet? Did it arise from a disturbing force foreign to itself, or
was it due to some intestine convulsion? Whence proceeded
202
DUPLICATION OF BIELA'S COMET.
the variations of brilliancy, too striking to be attributed to
optical illusion? When the separation was first observed, had
it already been accomplished some time ? We think with M.
Liais that this is likely, and that in four-and- twenty hours
the original comet could not have projected to a distance of
177 thousand miles a fragment which subsequently only
receded very slowly from the parent body.
Fig. 47. — The twin comets of Biela at their return in 1852. According to Secchi.
It may be readily imagined that astronomers were on the
look-out to again observe Biela's comet when it should return
in 1852. Accordingly, in August and September of that year
the two comets, which had performed their revolution in
company, were seen by Professor Challis at Cambridge, by
Father Secchi at Rome, and by M. Struve. This time the
mean distance of the two comets from each other was eight
times greater than on the occasion of the former passage, in
1846. The following table gives the distances between them
during the time they were observed : —
August 27,1852 . . . 1,502,000 miles.
September 4 „ . . . . . 1,560,000 „
12 „ . . ., . -. 1,603,000 „
20 „ . . . . . 1,624,000 „
28 „ . . "V . .: 1,615,000 „
268
THE WORLD OF COMETS.
It is to be remarked that the maximum distance of the
two nuclei corresponds, both in 1846 and 1852, to within a few
days of the date of the perihelion passage of the comets, which
took place in 1846 on February 12, and in 1852 on September
23 and 24.
The two comets, therefore, may be considered henceforth
as two distinct bodies ; and in fact, from their respective
movements have been obtained the elements of the two orbits,
which, however much they may resemble each other and be-
tray the community of their origin, exhibit none the less
marked differences. These elements, according to D'Arrest,
for the passage of 1852, were as follows : —
Elements of the two Cornels of Biela. .
d. h. m. d. h. re.
Perihelion passage, 1852— Sept. 24 5 14 p.m. ... 1852— Sept. 23 10 50p.m.
Longitude of perihelion . . 1092024 ... 1091321
Longitude of node . . . 246 5 16 ... 246 9 11
Inclination . . . . 12 33 25 ... 12 33 47
Perihelion distance . . . 0-860161 ... 0-860592
Eccentricity .... 0'7552007 ... 0-7561187
Period ..... 6 yrs. 214 days. 6 yrs. 229^ days.
Movement direct
These elements agree very closely, as may be seen, but
show a difference of 15^ days in the periods of the revolutions.
Have other perturbations been since experienced ? This was
a possibility sufficiently obvious to make the return of Biela's
comet a matter of doubt, if not in 1859, at least in 1866. In
1872 one of the nuclei of the comet was situated, at its
node, so near the earth that a rencontre between the two
may be conjectured to have actually taken place. To this
rencontre some astronomers have attributed the splendid
phenomenon witnessed by European observers on the night of
November 27 — the thousands of shooting stars which then fell
like a rain of sparks were, according to them, an integral part
'264
DUPLICATION OF BIELA'S COMET.
of one of these two comets. Other astronomers believe that
the phenomenon in question was due to the meeting of the
earth, not with one of the fragments of Biela's comet, but
with a swarm of meteors which had once formed part of the
same nebulosity. If the first conjecture be well-founded it
is not improbable that, under the powerful influence of the
earth's mass, the nebulosity of the comet has been completely
shattered. Future observations will perhaps furnish the
elements requisite for a solution of this problem.
[The circumstances connected with the passage of Biela's comet in 1872
were of so extraordinary a character that it seems desirable to give an account
of them here.
Professor Klinkerfues, of Gottingen, on comparing the brilliant meteor-
shower of November 27, 1872, with those of other years, -was led to the assump-
tion that in this instance we were in the closest proximity to Biela's comet.
Under these circumstances the comet would remain almost stationary in the
neighbourhood of the radiant of convergence for a few days after the meteor-
shower, and Professor Klinkerfues concluded that there was -even a hope of
finding the comet itself, provided the intelligence could be at once transmitted
to an observatory sufficiently far south. Accordingly, having determined the
radiant point from the tracks of eighty meteors, he sent the following telegram
to Madras : ' Biela touched earth on 27th, search near theta Centauri? This
telegram reached Madras by way of Russia in one hour and thirty-five minutes.
The consequences of it are best told in the words of Mr. Pogson, the Government
astronomer at Madras, who writes, under date December 5, 1872 : ' A startling
telegram from Professor Klinkerfues on the night of November 30 ran thus :
" Biela touched earth on 27th, search near theta Centauri" I was on the look-
out from comet-rise (16h) to sunrise the next two mornings, but clouds and
rain disappointed me. On the third attempt, however, I had better luck. Just
about 17^h mean time, a brief blue space enabled me to find Biela, and though
I could only get four comparisons with an anonymous star, it had moved for-
ward 28-5 in four minutes, and that settled its being the right object. I recorded
it as " Circular ; bright, with a decided nucleus, but no tail, and about 45" in
diameter." This was in strong twilight. Next morning, December 3, I got a
much better observation of it : seven comparisons with another anonymous star,
two with one of our current Madras catalogue stars, and two with 7734 Taylor.
This time my notes were " Circular ; diameter 75" ; bright nucleus, a faint but
distinct tail, 8' in length and spreading, a position angle from nucleus about
280°." I had no time to spare to look for the other comet, and the next morning
clouds and rain had returned.' For three mornings the sky was quite overcast,
265
THE WORLD OF COMETS.
and afterwards the comet would rise in daylight, and could not therefore be
observed.
The positions of the comet observed by Mr. Pogson do not well accord with
the calculated places of either part of Biela's comet, or of the meteor-stream
through which the earth passed on November 27. Capt. Tupman (R.A.S.
Notices, xxxiii. p. 318, March 1873) gives reason for his opinion, that the
body seen by Mr. Pogson was neither Biela's comet nor a meteoric aggregation
travelling in the same orbit, nor a body that had passed near the earth on
November 27. Dr. Oppolzer (Ast. Nach.,~Nos. 1920 and 1938, January 31 and
May 13, 1873), although he originally held the same view, was led by the
investigations he undertook to consider it highly probable that Pogson's
comet is closely connected with the shower of shooting-stars on Novem-
ber 27, and that it is even possible that it was a head of Biela's comet ; but
Dr. Bruhns (Ast. Nach., No. 2054, September 10, 1875) arrives at the con-
clusion ' that it is very probable that Pogson's comet was unconnected with
Biela's comet or with the meteor-swarm, and that it was a new comet.' If it
were Biela's comet, the latter must have been about twelve weeks behind
its time. It has been suggested that the observations made on December 2
and 3 referred to different heads of the comet, but there seems no doubt that
the body observed was the same on both occasions. In any case, the fact that
Professor Klinkerfues should have felt sufficient confidence in the truth of his
hypothesis to send the telegram to Mr. Pogson, and that the latter should have
actually detected a comet in the neighbourhood of the position indicated, forms
a very striking, I might almost say, romantic episode in astronomical history,
whether the body thus found was a portion of Biela's comet, or a meteor-swarm
on its track,- or even an independent body.
In consequence of the interest excited by the above observations, Mr. Hind
communicated to the Royal Astronomical Society (Notices, xxxiii. p. 320) an
account of the actual state of the calculations with regard to Biela's comet, from
which it appears that 'both nuclei of the comet were last observed in the autumn
of 1852, having been found much further from their calculated places than was
expected, a circumference which undoubtedly affected the number of obser-
vations, and which was occasioned by the unfortunate substitution by Professor
Santini of a semi-axis major depending wholly upon the observations of the pre-
vious appearance in 1846, in place of that which he had deduced from observa-
tion in 1832, and carried forward by perturbation to 1846. This source of
error in the prediction for 1852 is indicated by Professor Santirii in a communi-
cation made to the Venetian Institute in November 1854. There is no reason
to suppose that any perturbations beyond those resulting from known causes
operated between the appearances of the comet in 1846 and 1852 ; indeed, the
observations of these years have,been connected without difficulty by the appli-
cation of planetary perturbations during the interval.' The effect of the
perturbations was calculated by Professors Santini, Clausen, Hubbard, and
Michez, for the period from 1852 to 1866, and the perihelion passages were
266
DUPLICATION OF BIELA'S COMET.
fixed for May 24, 1859, and January 26, 1866. ' In 1859,' Mr. Hind proceeds,
' the position of the comet in the heavens rendered its discovery almost hopeless,
and its having passed by us unobserved is thus accounted for ; but it is not so
as regards the return in 1866. I believe it is certain that the comet did not
pass its perihelion in that year within seven weeks of the time predicted . . .
So far as I know at present the calculation of perturbations from 1866 to 1872
has not been undertaken by anyone ... it has probably been felt to be a use-
less labour to carry forward the elements from the predicted time of perihelion
in 1866, considering the want of success attending the endeavours to find
the comet in the corresponding track. ... If we suppose that the comet did
really encounter the earth [on November 27, 1872] in descending to perihelion
on December 27, there will be found since 1852 three mean revolutions of
6'754 years, and the perturbations being small from 1866 to 1872, the comet
might have been in perihelion about March 28, 1866, instead of January 26.
It is clear, therefore, that if the perihelion passage of Biela's comet took place
in 1866, six or eight weeks later than anticipated, its having passed unobserved
need occasion no surprise.' — ED.]
267
SECTION IV.
DOUBLE COMETS MENTIONED IN HISTORY.
Is there any example in history of the division of a comet into several parts ? — The
comet of B.C. 371 — Ephorus, Seneca and Pingre — Similar observations in Europe
and China— The Olinda double comet, observed in Brazil, in 1860, by M. Liais.
THE doubling of Biela's comet did not fail to direct attention
to the several instances on record of analogous phenomena
which had hitherto been looked upon as little worthy of belief.
It was then remembered that Democritus had, according to
Aristotle, related the fact of a comet having suddenly divided
into a great number of little stars. It was this, perhaps, that
gave rise to the opinion of certain philosophers of antiquity
that comets were composed of two or more wandering stars.
Seneca, in endeavouring to refute this opinion, mentions the
account given by Ephorus. the Greek historian, of the division
of the comet of the year B.C. 371 into two stars. He thus
expresses himself : —
' Ephorus, who is far from being an historian of unim-
peachable veracity, is often deceived — often a deceiver. This
comet, for example, upon which all eyes were so intently fixed
on account of the immense catastrophe produced by its ap-
parition— the submersion of the towns of Helice and Bura —
Ephorus pretends divided into two stars. No one but himself
has related this fact. Who could possibly have observed at
268
DOUBLE COMETS MENTIONED IN HISTORY.
what moment the comet dissolved and divided into two ? And
besides, if this division was actually seen to take place, how
is it that no one saw the comet form itself into two stars?
Why has not Ephorus given the names of these two stars?'
These two last arguments appear of little value, whilst the
fact itself mentioned by Ephorus, since the observations of
January 1846, appears no longer impossible.
In 1618 several comets were observed, some in Europe,
others in Persia, which could not be identified or distinguished
from one another with certainty. Two of these comets were
seen at the same time and in the same region of the heavens,
and this led Kepler to suspect they were parts of one and the
same comet, which had divided into two. When recording this
opinion of Kepler's, Pingre, who took part with Seneca against
Ephorus, now considers the great astronomer at fault, and
exclaims, ' Quandoque bonus dormitat Romerus I ' At the present
time, although unable to affirm that this division did actually
take place, we are forced to consider the conjecture of Kepler
as at least probable. There are, besides, other facts on record
not altogether dissimilar, and which are narrated by Pingre
himself : —
' In the year B.C. 14, Hantching-Ti ascended the throne of
China, in the twenty-sixth year of the fourth cycle ; in the
eighteenth year of his reign a star was seen to resolve itself into
tine rain and entirely disappear.1
1 Under the consulate of M. Valerius Massala Barbatus,
and P. Sulpicius Quirinus, before the death of Agrippa, a
comet was seen for several days suspended over the City of
Rome; it then appeared to resolve into a number of little
torches.' This is related by Dion Cassius.
According to observations recorded by the Chinese an-
nalists, and collected by Edward Biot, three comets joined
together appeared in the year 896, and described their orbits
269
THE WORLD OF COMETS.
iii company. Here again is a passage from Nangis, quoted
by Pingre, from which it appears that the comet of 1348
separated into several fragments: 'In the early part of the
night, in our presence pnd to our great astonishment, this
very 'large star divided into several beams, which spread
eastward over Paris and entirely disappeared. Was this
phenomenon a comet, or other star, or was it formed of
exhalations, and again resolved into vapour? These are
questions I must leave to the judgment of astronomers.'
It is very probable that some of the phenomena of sudden.
Fig. 48.— The Olinda double comet on February 27, 186D, according to M. Liais.
division mentioned by the ancients may have reference to
bolides or, as Pingre says, to ' meteors ' ; but the facts are
none the less curious, since between comets, bolides, and
shooting stars a real relation and community of origin have
been proved to exist.
But, before closing this section, we must not forget to
mention a phenomenon analogous, if not to the doubling of
Biela's comet, at least to the fact of two or more nuclei exist-
ing in the same comet. The observations were made at Olinda
in Brazil by M. Liais, a contemporary French astronomer, in the
270
DOUBLE COMETS MENTIONED IN HISTORY.
course of the months of February and March 1860. Fi"\
48 gives the aspect of the Olinda double comet on February
27, 1860, the day after its discovery; and the two following
figures 49 and 50 — which, like the first, reproduce the drawings
of M. Liais — suffice to define the positions and forms of the
two nuclei on the two latter dates. On February 27 the
principal comet, which was greatly superior both in size and
brilliancy to the secondary nebulosity, exhibited a nucleus from
which sprung two luminous sectors directed towards the
The nebulosity which formed the head enveloped these
sun.
Fig. 49. — The Olinda double comet on March 10, 1860, according to M. Liais
sectors, and, extending in the opposite direction, contributed
to form the tail, whilst the secondary comet consisted only of
a nebulosity preserving a marked condensation at its centre.
Twelve days after, on March 10, it had hardly undergone any
change, whilst the sectors had disappeared in the nucleus of
the principal comet, and were seemingly replaced by a nearly
circular envelope around the nucleus. On March 11 the
principal comet, instead of a condensation or single nucleus,
exhibited ' two other and smaller centres situated almost upon
the greater axis. The second nebulosity appeared of uni-
271
THE WORLD OF COMETS.
form intensity upon its circumference. It was much fainter
than the day before, and hardly visible. On March 11 there
was an evident tendency of the great nebulosity to divide, in
which case there would have been a triple comet/ On the next
day, March 12, the aspect had once more changed; a single
centre of condensation, situated like that of the 10th, was
visible, and the fainter of the two comets was with difficulty
distinguished. On March 13, the last day of observation, it
had completely disappeared.
Fig. 50. — The Olinda double comet on March 11, 1860, according to M. Liais.
The Olinda comet (the first comet of 1860), not being
periodical, or, what comes to the same thing, having a very
long period, it will not be possible to study the changes it will
in all likelihood have sustained at the very remote epoch of its
probable return. But astronomers, warned by the disruption
of Biela's comet, are now watching for the transformations of
these celestial embryos, these nebulous masses without consis-
tence, which the planets unsettle in their orbits, and which,
sometimes divided under the influence of a more powerful per-
turbation, or scattered in different fragments, as the ancient
272
DOUBLE COMETS MENTIONED IN HISTORY.
observations lead us to believe, shed throughout the regions of
space the matter of which they were originally formed.*
* Since writing the above lines a new comet [Coggia's comet, 1874] of con-
siderable interest, visible to the naked eye, has made its appearance in the
skies of Europe. In the last days of its visibility — too. brief, unfortunately —
the head appeared to undergo singular transformations, and to evince a certain
tendency to become double. Further on (Chap. X. sec. vii.) will be found the
particulars of these phenomena and drawings of the various appearances
presented.
273
CHAPTER IX:
MASS AND DENSITY OF COMETS,
x 2
SECTION I.
FIKST DETEKMINATION OF THE MASSES OF COMETS.
Lexell's comet, and the calculations of Laplace — The smallness of cometary masses
deduced from the fact that comets exercise no disturbing influence upon the
earth, the planets, or their satellites.
THE educated have long since ceased to believe in the mys-
terious influence of comets upon human events ; such a belief,
in fact, would imply a degree of superstition very little in
accordance with the spirit of modern times, and would denote
complete ignorance of astronomical phenomena. But if comets,
by their unexpected apparitions, no longer announce to the
world some great event or terrible catastrophe, are they not
capable of acting yet more directly for the overthrow of our
planet, either by disturbing it in its movement or by striking
against it in a rencontre which might prove fatal to its inhabit-
ants? We will further on consider the probability of such a
rencontre, and the effect it would produce upon our globe and
its inhabitants. But it is easy to understand that these effects
would very greatly depend upon two elements of which we
have not yet spoken, viz. the mass and density of the comet.
I have elsewhere * endeavoured to give an elementary idea
of the methods which astronomers have recourse to in order to
calculate the mass of any celestial body; that is to say, the
[* Le Ciel (' The Heavens '), part iv. An English translation, edited by
Mr. Lockyer, has been published by Messrs. Bentley & Co. — ED.]
277
THE WORLD OF COMETS.
quantity of matter it contains as compared with the mass of
the earth or of the sun ; in short, to weigh it. To this work,
therefore, I may be permitted to refer those of my readers who
are unfamiliar with astronomical determinations of the kind in
question. The methods employed differ from each other, but
all are based upon the principle of universal gravitation.
Let us now proceed to the results, and see what is known
of cometary masses.
We have seen that certain comets, in describing their orbits,
have approached sufficiently near to several of the planets,
Jupiter and Saturn, Mars and the Earth, to be sensibly dis-
turbed in their movements by the perturbations so produced.
These perturbations, the effect of which is to alter the form
and dimensions of the comet's orbits, have been predicted and
calculated beforehand; and the result has proved that the
accelerations and retardations assigned by theory were due, as
had been anticipated, to the disturbing action of the planetary
masses. If the masses of comets were of the same order of
magnitude as the planets themselves, they would reciprocally
cause an appreciable degree of change in the movements of
Jupiter or the other planets. Nothing of the kind has been
detected.
Let us take, for example, the comet of 1770 (LexelPs
comet), that famous comet which was compelled, in the first
instance, by the powerful attraction of Jupiter to describe an
elliptic orbit of short period, and, by a subsequent action of
the same planet, was consigned for ever to the depths of space.
Not only did this comet fail to exercise an appreciable influence
upon the mass of Jupiter at the two epochs of its passage in the
vicinity of the planet in 1767 and 1779, but it in no respect
disturbed any of its four satellites.* The same comet in 1770
* According to the calculations of Burckhardt, undertaken at the instigation
of Laplace, the comet in 1779 traversed the system of Jupiter's satellites, since its
278
FIRST DETERMINATION OF THE MASSES OF COMETS.
passed very near the earth ; its least distance from our globe
was but a sixtieth part of the distance of the earth from the
sun, viz., about 1,500,000 miles, or six times the distance of
the moon. Of all observed comets Lexell's comet has most
nearly approached the earth, which would have been sensibly
disturbed in its movement if the mass of the comet had been
at all comparable to that of our globe. ' Had the two masses
been equal,' says Laplace, ' the action of the comet would have
caused an increase of 11,612 seconds (centesimal) in the
length of the sidereal year.' We are certain, from the nu-
merous comparisons that Delainbre and Burckhardt made, in
order to construct their tables of the sun, that since the year
1770 the length of the sidereal year has not increased by 3''
(2"* 6 sexagesimal); the mass of the comet, therefore, was not
• part of that of the earth.*
distance from the planet was less than the mean radius of the orbit of the furthest
satellite. But it follows from the researches of M. Le Verrier, published in 1844,
that the distance was in reality equal to at least three and a half times this
radius. The conclusions, therefore, that were obtained relative to the small
mass of the comet are not justified.
* ' Not only,' says Laplace elsewhere, ' do comets fail by their attraction to
disturb the movements of the planets and their satellites, but if, as is very pro-
bable, in the course of past ages any comets have come in contact with these
bodies, the shock of the rencontre would not appear to have exercised much
influence upon their movements. It is difficult not to believe that the orbits of
the planets and their satellites were nearly circular in the beginning, and that
their small eccentricities, as well as the common direction of their movements
from west to east, depend upon the initial circumstances of the solar system.
Neither the action of comets nor collisions with them have changed these
phenomena ; and yet, if any comet meeting with the moon, or with one of
Jupiter's satellites had had a mass equal to that of the moon it would in all j
probability have rendered their orbits very eccentric. Astronomy presents two
other very remarkable phenomena, dating apparently from the origin of the
planetary system, and which a very moderate shock would have destroyed
entirely : I mean the equality of the movements of rotation and revolution of
the moon and the librations of the first three satellites of Jupiter. It is very
evident that the blow of a comet whose mass did not exceed the thousandth
part of that of the moon would suffice to give a very sensible value to the real
librationa of the moon and the satellites. We may, therefore, be reassured as
279
THE WOULD OF COMETS.
to the influence of comets, and astronomers have no reason to fear that they can
in any respect interfere with the accuracy of astronomical tables.' — Me'canique
Celeste, t. iv. p. 256.
There is, however, in the planetary system an anomaly which might be con-
sidered as arising from the perturbations due to a rencontre with a comet. We
mean the great inclination and the retrograde movement of the satellites of
Uranus. Such an hypothesis appears to us not improbable ; and it would in-
validate the conclusions drawn by Laplace from the uniformity and the constancy
of the motions of the planets and their satellites.
[No doubt the anomalous motion of the satellites of the distant planet Uranus
weakens somewhat the force of Laplace's argument, but not, it seems to me,
to any serious extent. Laplace's arguments, in regard to the moon's rotation
and the librations of Jupiter's satellites, remain of course unaffected. — ED.]
SECTION II.
METHOD OF ESTIMATING THE MASSES OF COMETS BY
OPTICAL CONSIDERATIONS.
The masses of Encke's comet and the comet of Taurus determined by M. Babinet —
Objections to this method of determination.
WE have thus a determination of cometary masses deduced
from the reciprocal disturbances exercised by comets and the
planets on one another. It shows that comets have extremely
small masses, since, greatly disturbed themselves in their
course when they approach a planet, they appear never to have
exercised any disturbing influence upon the movements of the
planet itself. But, from the value found for the mass of
Lexell's comet — a value which, however, is only a maximum
limit — it may be seen how far a comet is from being considered
a visible nonentity (rien visible), to make use of the forcible
expression of M. Babinet. The 5,000th part of the mass of
the terrestrial globe is equivalent to the sixtieth part of the
mass of the moon, a quantity, it will be agreed, far from
negligible.
For the justification of his expression M. Babinet has
relied upon the following optical considerations. He has
called attention to the known fact that stars of exceedingly
faint light may be seen through cometary nebulosities without
their light losing any of its intensity. Considering, for
281
THE WORLD OF COMETS.
example, the comet of Encke, which in 1828 had the appearance
of a globe-shaped nebulous mass of 311,000 miles in diameter,
and through which Struve saw, without any apparent diminution
of lustre, a star of the eleventh magnitude, M. Babinet reasons
as follows:— The cometary nebulosity having in no respect
altered the luminous intensity of the star, we may conclude
that its intensity could not have been the sixtieth part of that
of the star. Now, the atmosphere illuminated by the full moon
obliterates all stars of less than the fourth magnitude, and yet
the lunar light has, according to Wollaston, an illuminating
power 800,000 times less than that of the sun. Lastly, taking
into consideration the relative thicknesses of our atmosphere and
of the comet, M. Babinet has arrived at this conclusion: that
the substance of a comet is of no greater density than that of
our atmosphere divided by the enormous number forty-jive
thousand billions. According to this reckoning Encke's comet
would hardly weigh twelve hundred tons.
The same method of estimating cometary masses by optical
considerations has also been applied by M. Babinet to the
cornet of 1825, the so-called comet of Taurus. We have seen
that the comet, when interposed before a star of the fifth mag-
nitude, altered its brightness in no perceptible degree. The
star in question had, therefore, not lost more than half a mag-
nitude, or about a fifth of its light. It had consequently pre-
served at least four- fifths of its normal brightness. Now, its
light was then traversing a stratum of about 5,000 miles in
thickness; that is to say, a thousand times the thickness of the
atmosphere, supposing it to be throughout of the density of
the air at the surface of the earth. And as it is known that
light in traversing perpendicularly our atmosphere loses more
than a quarter of its intensity, it follows that the brightness of
the star must have been reduced to the fraction ( J)1)00° of its
, real brightness, if the density of the cometary nebulosity were
282
OPTICAL METHOD OF DETERMINING THE MASSES OF COMETS.
the same as that of the air. This density is, therefore, enor-
mously less, and it is expressed by a fraction having unity for
its numerator, and for its denominator a number consisting
of 126 figures. ' When,' he says, in conclusion, ' Sir John
Herschel, in his last work upon astronomy, spoke of a few
ounces as the mass of a comet's tail, his statement was received
with almost general incredulity. Nevertheless this estimate is
quite exaggerated in comparison with the preceding.'
We will not seek to enquire if the calculations to which
these ingenious methods lead are based upon data beyond
all dispute, if the density is proportional to the absorption of
light, and if the substance of which cometary nebulosities are
formed is comparable to that of known gases, both in respect
to their molecular composition and respective optical proper-
ties. But, granting the conclusion to be legitimate, it must be
noticed that it is one which applies only to the comet of 1825
and to that of Encke, or at most to comets of no higher lumi-
nous intensity. The whole argument of M. Babinet depends
upon the extremely feeble intensity of cometary light as com-
pared with the illumination of the atmosphere by the sun, and
the great extent of the nebulosity traversed by the stellar
light. This reasoning, therefore, does not hold good in respect
to very luminous comets — those, for example, which have been
seen at noonday and in sunshine with the naked eye — such as
that of the year B.C. 43, and those of the years 1006, 1402,
1532, 1577, 1618, 1744, and especially the great comet of 1843,
which was observed at Florence at noonday, at 1° 23' distance
from the sun. The first comet of 1847 was visible at London
in the vicinity of the sun. Even if we set aside these as
exceptionally brilliant comets, we have seen that the obser-
vations of the 5th comet of 1857, on September 8, were in
TIO respect obstructed by the light of the moon. Such
comets are not to be compared with Encke's comet, a feeble
283
THE WORLD OF COMETS.
nebulosity, with hardly any central condensation.* Besides, it
is not certain that the stars which have been seen through
cometary nebulosities would not have been changed in their
intensity, perhaps even eclipsed, if the occultation, instead
of taking place behind some portion of the nebulosity, had
occurred strictly behind the nucleus, the most luminous por-
tion of the head of the cornet. No occultation of this kind
has yet, to our knowledge, been proved with certainty to have
taken place.f It would therefore be wrong to generalise upon
the foregoing conclusion, for, whilst everything leads us to
believe that cometary masses are in general greatly inferior to
the planetary masses, there is nothing to prove that certain
amongst them may not attain a value sufficiently great to
produce, in the event of a rencontre with the earth, or with
any other planet, a shock or some other kind of sensible
perturbation.
' This condensation, however, has been sometimes much less feeble. M.
Faye remarks : ' The relative density of Encke's comet must be pretty consider-
able, since it can appear to the naked eye as a star of the fourth magnitude.'
t [See Chapter X. sec. ii. p. 294.— ED.]
284
SECTION III.
THIRD METHOD OF DETERMINING THE MASSES OF COMETS.
Theory of the formation and development of cometary atmospheres under the influ-
ence of gravitation and a repulsive force — Calculations of M. Edouard Roche —
Masses of the comets of Donati and Encke as determined by this method.
WE are now about to see the same question, when investigated
by another method, lead to results quite different to those of
M. Babinet.* Between the opinions — entirely conjectural, be it
observed — of the savants of the eighteenth century who held
that comets were bodies dense and massive as the planets, and
those of some contemporary astronomers who regard them as
visible nonentities, there is room for a determination which
is removed from both extremes, and is moreover better
justified.
For this method of determination we are indebted to M.
Edouard Roche, professor in the Faculty of Sciences at Mont-
pellier. In a series of very remarkable researches into the
theory of cometary phenomena, which we shall analyse further
on, M. Roche shows that there is a determinate relation
* The following is the passage from the Outlines of Astronomy, to which
Babinet alludes (ante, p. 283) : ' Newton has calculated (Princ., iii. p. 512) that
a globe of air of ordinary density at the earth's surface of one inch in diameter,
if reduced to the density due to the altitude above the surface of one radius of
the earth, would occupy a sphere exceeding in radius the orbit of Saturn. The
tail of a great comet, then, for aught we can tell, may consist of only a very few
pounds or even ounces of matter.' But Herschel, it will be noted, speaks only
of tails, not of atmospheres and nuclei.
285
THE WORLD OF COMETS.
between the distance of the comet from the sun, its mass, and
the diameter of the portion of its nebulosity subject to the
attraction of the nucleus, otherwise called the diameter of its
true atmosphere. This relation holds at distances so remote
from the sun that the repulsive force, either apparent or real,
which engenders the tail may be neglected. Another element
the repulsive force — comes into operation when the comet
approaches the perihelion; or rather when the comet is near
the sun, it is necessary to take account of this force.
Relying, then, upon micrometric observations, which fur-
nish an approximate estimate of the diameters of the nebulo-
sities of the comets of Encke and Donati, M. Roche has arrived
at the following result.
As compared with the mass of the earth the mass of
Donati's comet would be equal to U'000047; that is to say, to
about the twenty-thousandth part of the former, or about
fifty-three times the mass of the terrestrial atmosphere. It
would be equal in weight to a sphere of water of 250 miles
radius, or to about 268,000 billions of tons — a very different
estimate indeed to the pounds of M. Babinet ! As regards the
density, M. Roche deduces it from the diameters of the nu-
cleus and the nebulosity, which in October 1858 were nearly
4" and 50", or 990 miles and 12,400 miles respectively.
Assuming that the mass remained unchanged from June, the
date of the first determination, to the month of October, and
that the mass of the nebulosity was the 1,000th part of
the total mass, the density of the nucleus would be almost an
eighth of that of water, and the density of the nebulosity
about the 154,000th part of that of atmospheric air.
The mass of Encke's comet, estimated by the same method,
is found to be about the 1,000th part of the mass of the earth.
' Although we have found,' says M. Roche, ' for the comet of
Encke a mass superior to what might have been supposed,
286
THIRD METHOD OF DETERMINING THE MASSES OF COMETS.
a priori, these numbers are not inadmissible, and it seems
to us that no serious objection can be made to our
theory.'
Of the three methods for the determination of cometary
masses which we have just passed in review the first is the
most certain; but it has furnished as yet only negative solu-
tions of the problem, and these few in number. It leads us
to believe that the masses of comets are very small in com-
parison with the planetary masses ; and it is from the absence
of all perturbation caused by comets that we have been able
to deduce a superior limit to their masses. The second method,
founded upon optical considerations, is the most conjectural,
because it assumes that the transparency is inversely as the
density, an hypothesis entirely gratuitous, considering how
completely ignorant we are of the true physical condition of
the substance of which comets are composed. To the third
method, therefore, it seems to us the preference should be
given, and it is this, in fact, which has furnished the most
positive results.* But the subject, it cannot be denied, con-
tinues to be involved in much obscurity.
The foregoing remarks, it must be borne in mind, have
reference only to the mass of comets. In speaking of their
density it would be evidently necessary to distinguish between
the nucleus, either solid or liquid, when sufficiently distinct,
and the nebulosity of the comet. To the density of this nebu-
losity, or that of the tail, the calculations and results of M.
Babinet might reasonably be applied. The density of the
nucleus could, it is true, be easily deduced from the mass of the
comet, if we neglect the mass of the matter that envelopes it;
[* I cannot refrain from expressing my own opinion that the first method
(viz. by means of the perturbations produced by comets) is the only satisfactory
one. The other two both involve hypotheses and assumptions which render the
results obtained by means of them most uncertain. — ED.]
287
THE WORLD OF COMETS.
but for this we should require very exact measurements of the
nucleus, which would be difficult to obtain. It is known, more-
over, that the dimensions of the nucleus vary in the same comet
with the distance of the comet from the sun. The density, there-
fore, must itself vary with the distance and these dimensions.
288
CHAPTEE X.
THE LIGHT OF COMETS,
u
SECTION I.
INTEREST ATTACHING TO THE PHYSICAL STUDY OF COMETARY
LIGHT.
WE have seen what the telescope has taught us of the structure
of comets, so complex and wonderfully mobile, so different in
this respect from that of the planets or the sun. On the one
hand we see solid or liquid bodies, bearing the most striking
analogy to the terrestrial globe, surrounded like it by atmos-
pheres of comparatively small extent, stable in every portion ;
these are the planets, the moon, and the satellites of the
planets. As regards the sun and the stars — which shine, like
the sun, by their own light, and are, like him, as everything
leads us to suppose, foci of light and heat to other planetary
groups — if these bodies are incandescent gaseous masses, their
condensation is so enormous and their physical constitution is
comparatively so stable, that the changes of which they are
perpetually the theatre have no appreciable effect upon their
equilibrium. In comparison with comets they are permanent
stars ; while comets seem to be nothing more than clouds — wan-
dering nebulaB, to employ the expression of Laplace, who has
but reproduced in a more happy form the term so happily
applied by Xenophanes and The.on of Alexandria.
But it is not merely by its concentration in the field of a
telescope that the light of a comet may be made subservient
291 TT 2
THE WORLD OF COMETS.
to the study of its physical constitution ; the undulations of
which it is composed, after passing through the depths of
space and arriving at the confines of the atmosphere, after
passing through the atmosphere and penetrating the crystal
lens of the instrument, retain certain distinctive qualities by
which the savant who subjects it to analysis may distinguish
whether this light has emanated directly from the body itself
or, on the contrary, has undergone reflexion within the co-
metary mass, and is consequently only light reflected from the
sun. Other methods of analysis will permit us to penetrate
yet more deeply into the inmost constitution of a comet and
its different parts, and the light with which it shines is again
the agent to which we have recourse, and which will reveal to
us the che'mical nature of the cometary matter. So that only
one more difficulty remains to be surmounted in order to
unveil completely the composition of these once mysterious
bodies ; that is, to penetrate actually and materially into the
interior of a cometary mass. And, as we shall shortly see,
there is great probability of such an event being realised, if it
has not already partially happened. In any case we have
already said enough to show the interest attaching to the study
of cometary light, the subject of the following sections of this
chapter.
202
SECTION II.
TRANSPARENCY OF NUCLEI, ATMOSPHERES, AND TAILS.
Visibility of stars through the atmospheres and tails of comets ; ancient and modern
observations upon this point — Are the nuclei of comets opaque, or transparent
like the atmospheres and tails ? — .Reported eclipses of the sun and moon pro-
duced by comets.
THE visibility of stars, even of very small ones, through the
coma3 and tails of comets is a fact which had been observed by
the ancients. Aristotle in his Meteorology mentions the stars
seen by Democritus notwithstanding the interposition of a
comet. Seneca says likewise, in his Qucestiones Naturales,
* that we may see stars through a comet as through a cloud ; '
and further on, ' the stars are not transparent, and we can see
them through comets — not through the body of the comet
where the flame is dense and solid, but through the thin and
scattered rays which form the hair ; it is through the intervals
of the fire, not through the fire itself, that you see.' Humboldt,
in quoting this last passage, * per intervalla ignium, non per
ipsos vides,' adds : ' This last remark was unnecessary, for it is
possible to see through a flame the thickness of which is not too
great.' This is true ; but Seneca has merely recorded the fact
that up to his tune stars had been seen behind the tail or
coma, not behind the nucleus itself. The want of the telescope
did not, in fact, permit the ancients to distinguish the body or
nucleus of a comet, even when the comet had a nucleus.
293
THE WORLD OF COMETS.
Modern astronomers themselves are not in a better position,
since all observed occupations of stars by comets, one alone ex-
cepted, refer to the interposition of the nebulosity forming the
coma, not to that of the nucleus properly so called. We here
mention the principal instances observed, and more especially
those which have been invoked to prove the transparency of
cometary light, beginning with the single exception above
referred to, of which Arago gives the following account : ' On
the 27th of October, 1774, Montaigne saw at Limoges a star
of the sixth magnitude in Aquarius through the nucleus of a
small comet.' * Let us now proceed to the others.
On November 9, 1795, Sir William Herschel distinctly
perceived a double star of the eleventh or twelfth magnitude
through the central part of the nebulosity of a comet. The
two component stars, one of which was much fainter than
the other, were both clearly visible. The comet was Encke's,
which is generally destitute of nucleus, and very rarely ex-
hibits more than a faint condensation of light in the centre of
its nebulosity. On November 7, 1828, Struve saw in the
centre of the same comet a star of the eleventh magnitude,
which for a moment he mistook for a cometary nucleus,
and whose brightness appeared in no respect diminished. Now,
the thickness of the nebulosity interposed was not less than
310,000 miles. This is the observation upon which, as we
have seen, M. Babinet has founded his calculation of the mass
and density of the nebulosity itself. According to an observa-
* [I myself saw the nucleus of Halley's comet at its apparition in 1835 pass
over a star, when I was at the Cambridge Observatory. I remember the circum-
stance distinctly, and my impression is that there wag no diminution at all in the
brightness of the star. The printed record of my observation runs as follows :
' Sept. 25, 9h 45m to 12h. During the whole time the comet' (seen with the
equatorial 3| inch aperture) appeared to continue changing its figure. It passed
over three stars (the nucleus covering one), which were distinctly visible during
the whole time.' — Cambridge Observations, vol. viii., 1835, p. 216. — ED.]
294
TRANSPARENCY OF NUCLEI, ATMOSPHERES, AND TAILS.
tion made at Geneva twenty-one days later by M. Wartmann
a star of the eighth magnitude was, on the contrary, com-
pletely eclipsed by the comet. It is interesting to compare
these two observations, which show the comet's condensation
between the dates mentioned ; in this interval the volume of
the nebulosity had become reduced to one-eighth, and there
must have been a corresponding luminous condensation and
increased brilliancy, which would explain the occultation seen
by Wartmann. In April 1796 Olbers remarked a similar fact
in respect to a star of the sixth or seventh magnitude, which,
hardly weakened in intensity, appeared a little to the north
of the centre of the nebulosity ; the star, therefore, was
not occulted by the nucleus, but its light was sufficiently
bright to render the nucleus for some time invisible in its
vicinity.
Cacciatore observed, at Palermo, the occultation of a star
by the comet of 1819. ' On the 5th of August,' he states, ' I
observed through the nebulosity, very close to the nucleus, a
star which at the most was of the tenth magnitude.'
When we add to these observations that of Struve, who,
on October 29, 1824, saw a star of the tenth magnitude at 2"
from the centre of the comet, without the light of the star
being at all diminished ; those of Pons and Valz, in 1825, who
saw, the former a star of the fifth magnitude, and the latter
one of the seventh magnitude occulted by the famous comet of
Taurus, it will be seen that the light of comets, not only that
of their tails, but also that of their nebulosities in close prox>
imity to the nucleus, is transparent in the highest degree.
But is the nucleus properly so called equally transparent?
This we have not yet data to determine, since we have no
observation of the occultation of a star by a comet, which
indicates with certainty the interposition of the nucleus,
excepting that mentioned by Montaigne. Pons,^ in 1825,
295
THE WORLD OF COMETS.
mentions the passage of the star to the centre of the nebulosity,
but not its passage behind the luminous nucleus.
From the foregoing facts we are forced to conclude that
the matter of cometary tails and nebulosities, if gaseous, is of
extreme tenuity ; but it is perhaps so discrete — i.e. the parti-
cles are so far apart — as not to occasion any perceptible occul-
tation of a light seen through them. This was the opinion of
M. Babinet, who, from the calculations above quoted respecting
this extreme tenuity of cometary matter, has come to the con-
clusion that ' the substance of comets, therefore, is a kind of
very divided matter, consisting of isolated particles, without
mutual elastic reaction,' An observation made by Bessel
helps to confirm this view, as Humboldt, when recording it,
justly remarks. On September 29, 1835, Bessel saw, about
8" from the centre of the head of Halley's comet, a star of the
tenth magnitude. At this moment its light was traversing a
considerable portion of the nebulosity ; but the luminous ray
was not deflected out of its rectilinear course, as the illustrious
astronomer satisfied himself, by measuring the distance be-
tween the occulted star and a star visible on the verge of,
but outside, the nebulosity. ' So complete an absence of re-
fracting power,' says Humboldt, ' scarcely allows us to suppose
that the matter of comets is a gaseous fluid. Must we have
recourse to the hypothesis of a gas infinitely rarefied, or are we
to believe that comets consist of independent molecules, the
union of which constitutes cosmical clouds, devoid of power to
act upon the luminous rays that pass through them, just as the
clouds of our own atmosphere do not alter the zenith distances
of the stars ? '
It, therefore, still remains an open question whether the
cometary nucleus, the luminous and brilliant central portion
of a comet — that part of it, in short, which gives to the comet
the appearance of a star — is opaque or transparent. In any
296
TRANSPARENCY OF NUCLEI, ATMOSPHERES, AND TAILS.
case, let us repeat, it is clear that we must refrain from general-
ising, for it would be absurd to identify, from this point of view,
the faint nuclei, hardly visible in the telescope, of many of the
smaller comets with those of comets which have shone like stars
of the first magnitude, and have been luminous enough to ap-
pear in broad daylight and shine in the most brilliant regions of
the heavens in the vicinity of the sun.
In support of the opacity of cometary nuclei various
anciently recorded facts have been adduced ; but these facts
are either apocryphal or at least very doubtful. Thus the
eclipse of the year B.C. 480, mentioned by Herodotus, and the
eclipse mentioned by Dion Cassius, which took place in the
year in which Augustus died, not admitting of explanation
from the movement and interposition of the moon, were sup-
posed to be due to the intervention of comets, a supposition
altogether without foundation and very improbable. In the
Cometographie of Pingre', under the date of 1184, we find the
following : ' On the 1st of May, about the sixth hour of the day,
a sign was visible in the sun : its lower portion was totally ob-
scured. In the centre it was traversed by what appeared to be
a beam! The rest of its disc was so pale that it impressed the
same pallor upon the faces of those who looked upon it. Was
this phenomenon the effect of a comet situated between the sun
and ourselves? I do not know, but I consider it possible.'
The total obscuration of the lower part of the sun would be,
on this hypothesis, the partial eclipse produced by the opaque
nucleus, and the beam traversing the disc the densest portion
of the tail. Lastly, the pallor of the sun could be explained by
the interposition of the vapours composing the nebulosity. But
this is mere supposition,
Not an eclipse of the sun but an eclipse of the moon would
appear to have been caused by the comet of 1454, according to
Phranza, the protovestiare, or master of the wardrobe, of the
297
THE WOELD OF COMETS.
Turkish emperors. But it has been proved that the Latin
version of the text is corrupt, and that Phranza has simply
chronicled the fact of the simultaneous presence in the heavens
of the comet and the full moon at the time when the latter was
eclipsed.
298
SECTION III.
COLOUR OF COMETAftY LIGHT.
Different colours of the heads and tails of comets — Examples of colour taken from
the observations of the ancients : red, blood-red, and yellow comets — Difference
of colour between the nucleus and the nebulosity — Blue comets — The diversity of
colour exhibited by comets is doubtless connected with cometary physics, and
with the temperature and chemical nature of cometary matter.
THE light of many comets has been sensibly coloured. The
comet of B.C. 146 exhibited a reddish tinge, according to
Seneca : ' A comet as large as the sun appeared. Its disc
was at first red and like fire.'...
A little further on Seneca again observes : l Comets are in
great number, and of more than one kind ; their dimensions are
unequal, their colours are different; some are red, without
lustre ; others are white and shine with a pure liquid light.
...Some are blood-red, sinister presage of that which will
soon be shed.' The ancients had, therefore, observed the
difference of colour in the light of comets. And we shall
mention a number of similar examples taken from the chronicles
of the Middle Ages and from modern observers.
The comets of 662 and 1526 are cited by Arago as having
been ' of a beautiful red ; ' and we have seen that Pliny in his
classification speaks of comets whose ' mane is the colour of
blood.' Such was the comet which appeared in November
1457; according to an ancient chronicle 'its coma or tail
299
THE WOULD OF COMETS.
resembled the colour of flame.' The horrible comet which,
according to Comiers, appeared in 1508 was very red, repre-
senting human heads, dissevered limbs, instruments of war,
&c.' The first comet of 1471 'was very large and of a
reddish colour ; it rose before the dawn.' In 1545 'a comet
whose coma was the colour of blood burned for several days ;
it then became pale and soon disappeared.' Gemma, when
speaking of the comet of 1556, thus expresses himself:
* Although Paul Fabricius has stated that the comet appeared
small to him, I can affirm that, from the commencement of its
apparition, I found it not less than Jupiter in size ; the colour
of the comet resembled that of Mars ; its ruddy colour, how-
ever, degenerated to paleness.' This remark refers more espe-
cially to the nucleus, for, according to another eyewitness, ' the
colour of the tail towards its extremity continued pale, livid,
and similar to that of lead.' The opposite was the case with
the comets of 1577 and 1618. Tycho relates of the first that
its head was round, brilliant, and remarkable for a certain
leaden whiteness, whilst the tail, turned towards the east,
darted in a direction opposite the sun rays of a more ruddy
colour. As regards the second, its tail appeared, says Arago,
of a very bright colour.
The comet of 1769 had a slightly reddish nucleus, as also
had that of 1811, observed by Sir William Herschel ; but the
nebulosity of the latter was of a bluish green, which caused
Arago to conjecture that this appearance of colour might be
due to the simple effect of contrast. It is clear, however, that
the colour of the nucleus and that of the nebulosity were very
sensibly different. A brilliant zone, narrow and semicircular,
surrounding the head of the comet of 1811 on the side nearest
to the sun, was of a decided yellow colour.
Amongst observations of earlier date we find mention
made of comets which have shone with a golden yellow light.
300
COLOUR OF COMETARY LIGHT.
Such was the comet of 1555, whose rays shone like gold ; and
that of 1533, whose tail was of a beautiful yellow. Of Halley's
comet, at its apparition in 1456, it is said ' the colour of the
comet resembled that of gold.' It is true that ' at other times
and perhaps in other places it appeared pale and whitish ; it
sometimes resembled a glistening flame.'
This last remark suggests a very natural reflection, and
leads us to consider how far the state of the atmosphere, its
more or less purity, and the greater or less height of the comet
above the horizon, may have contributed to invest these bodies
and their tails with the tints above described. It appears
certain, however, that the light of comets is far from being
always of the same colour. The great comet of 1106 was of a
remarkable whiteness. ' Situated towards that quarter of the
heavens where the sun sets in winter, it extended a whitish
beam, resembling a linen cloth. From the commencement of
its apparition both the comet and its beam, which was as
white as snow, diminished day by day.' According to other
chronicles ' its rays were whiter than milk.' This, as may be
seen, forms a complete contrast to the red and yellow colours
of the preceding comets ; nor is the contrast less with the
comets of which we are about to speak.
Under the date of 1217 Pingre' has the following: ' Several
prodigies were observed ; blue comets were seen.' The comet
of 1356, observed in China, was of a whitish colour bordering
upon blue. The comet of 1457, the tail of which resembled
an upright spear, was of 'a livid dusky colour, very like that of
lead.' The second comet of 1468 also * was blue, but some-
what pale.' The one which appeared at the end of 1476 was
of pale blue bordering upon black. And we must not forget
the two comets, * terrible and of a blackish hue,' whose appari-
tion in 1456, before that of Halley's comet, has been mentioned
by some authors,
301
THE WORLD OF COMETS.
Modern observers appear to have paid but little attention
to the study of cometary light. Nevertheless, we find in
Arago the following comparison between the tail of the great
comet of 1843 and the zodiacal, light: ; On the 19th of March
the tail of the comet, situated close to the zodiacal light, was
evidently tinged with red, inclining to yellow.' He says no-
thing about the colour of the nucleus. Amongst the numerous
observations of the comet of Donati (1858) we have only
met with the following mention of its colour, made by an
observer at Neufchatel, M. Jacquet : ' On Sunday, the 3rd of
October, after a cloudless day, splendid twilight. The irre-
gular line of mountains near where the sun has disappeared
is traced against a sky glowing with gold and fire. It is six
o'clock. We endeavour to see if the comet, in consequence of
the purity of the air, may not be already visible. After a few
moments' search we discover it, extremely small and pale, and
of the silvery brightness of a planet seen by daylight.' Two
days after, on October 5, at the same hour, the comet was
visible in the neighbourhood of Arcturus. ' The clouds,' says
M. Jacquet, ' pass from the region of Arcturus far too slowly
for my patience ; they disperse at last ; I see a yellow star,
and a little underneath and to the right a small white plume.
My attention is caught by these two colours ; one would say
the comet is of gold and the plume of silver ' (Souvenirs de
la Comete de 1858). This evidently refers to the colour of
the tail and the envelope surrounding the nucleus, for the
next day the same observer speaks of the nucleus as ' small,
bright, and of a reddish yellow.'
Coggia's comet (1874, III.), observed this summer, was
distinguished by very appreciable phenomena of colour. Fa-
ther Secchi remarks : ' The comet, when observed with an
ordinary eyepiece, was magnificent. On the 9th of July it
302
COLOUR OF COMETARY LIGHT.
formed a fan, of a reddish tint (by contrast), of about 180 de-
grees of opening, composed of curvilinear rays, springing from
a nucleus of yellowish green.' The Roman astronomer thus
attributes the colour of the tail to the effect of contrast. M.
Tacchini is of a different opinion. After having described
the continuous spectrum upon which was projected the dis-
continuous spectrum of the nucleus, he proceeds to add : * This
beautiful coloured band, which presented itself only at the
passage of the nucleus, when seen through a simple eyepiece
appeared of a greenish white, whilst the fan itself was sensibly
reddish, even when occulting the nucleus.' *
The question of the colour of the light in the several
portions of a comet, its nucleus, atmosphere, and tail, is an
interesting one, for it is intimately connected with the physical
nature of the light itself. In conjunction with results afforded
by spectroscopes and polariscopes it will doubtless help to
determine if comets shine by their own light, or only reflect
* ' Questo bel nastro colorato presentavasi al solo passagio del nucleo, il quale
guardato coll' oculare semplice appariva bianco- verdastro, mentre il ventaglio era
sensibilmente roseo anche occultando il nucleo.' (Memorie delta Societa degH
Spettroscopisti Italiani. Luglio, 1874.)
[Mr. Huggins, describing the appearance of the comet in the telescope,
writes, ' The nucleus [of Coggia's comet] appeared of an orange colour. This
may be due in part to the effect of contrast with the greenish light of the coma.
Sir John Herschel described the head of the comet of 1811 to be of a greenish
or bluish-green colour, while the central point appeared to be of a pale ruddy tint.
The elder Strube's representations of Halley's comet, at its appearance in 1835,
are coloured green, and the nucleus is coloured reddish yellow. He describes
the nucleus on October 9 thus, " Der Kern zeigte sich wie eine kleine, etwas
in gelbliche spielende, gliihende Kohle von langlicher Form." Dr. Winnecke
describes similar colours in the great comet of 1862 ' (Proc. Roy. Soc., vol.
xxiii., p. 157). According to Mr. Lockyer, the colour, both of the nucleus and
of the head, as observed in Mr. Newall's telescope, was a distinct orange yellow.
Mr. Newall says the colour of the comet was greenish yellow. Messrs. Wilson
and Seabroke, observing the comet on July 14, at Rugby, considered that it was
reddish in colour (R.A.S. Notices, vol. xxxv., p. 84). — ED.]
303
THE WORLD OF COMETS.
that which they receive from the sun. Perhaps both hypotheses
are true; but if so, to what extent do the atmosphere and the
nucleus participate in this double cause of visibility ? This is
a question we are not yet in a position to answer, although,
as we shall see, several steps have already been taken in this
direction.
SECTION IV.
SUDDEN CHANGES OF BHILLIANCY IN THE LIGHT OF COMETARY
TAILS.
Rapid undulations occasionally observed in the light of cometary tails ; observations
of Kepler, Hevelius, Oysatus, and Pingre ; comets of 1607, 1618, 1652, 1661, and
1769 — Undulations in the tails of the comets of 1843 and 1860 ; do these undu-
lations arise from a cause peculiar to the comet itself, or do they depend upon
the state of the atmosphere ? — Objection made by Olbers to the first of these
hypotheses ; refutation by M. Liais.
THE tails of certain comets have exhibited variations'of brilli-
ancy, sudden changes of intensity, analogous to the phenomena
of the same kind which are observed in the aurora borealis,
and which, it is believed, have been remarked in the zodiacal
light. This fact was unknown to the ancients ; and when
Seneca speaks of the augmented or diminished brilliancy of
comets, it is evident that he alludes to the changes produced,
in the course of their apparition, by the variations of their
distance from the earth. He compares them 'to other stars
which throw out more light and appear larger and more
luminous in proportion as they descend and come nearer to us,
and are smaller and less luminous as they are returning and
increasing their distance from us.' (Qucestiones Naturales,
vii. 17.)
Kepler is the -first observer* who has made mention of
* There are, however, some earlier observations of the same fact. The tail of
the comet of 582 appeared, according to Gregory of TOUTS, like the smoke
305 X
THE WORLD OF COMETS.
these singular changes. ' Those/ he says, ' who have observed
with some degree of attention the comet of 1607 (an appari-
tion of Halley's comet), will bear witness that the tail, short
at first, became long in the twinkling of an eye.1 Several
astronomers, Kepler, Wendelinus, and Snell, *aw in the
comet of 1618 jets of light, coruscations and marked undu-
lations. According to Father Cysatus the tail appeared as if
agitated by the wind ; the rays of the coma seemed to dart
forth from the head and instantly return again. Similar
movements were observed by Hevelius in the tails of the
comets of 1652 and 1661; and Pingre, describing the obser-
vations of the comet of 1769, made at sea, between August 27
and September 16, by La Nux, Fleurien, and himself, thus
describes the phenomenon of which he was a witness : ' I
believe that I very distinctly saw, especially on September 4,
undulations in the tail similar to those which may be seen in
the aurora borealis. The stars which I had seen decidedly
included within the tail were shortly after sensibly distant
from it.'
M. Liais has given the following account of the obser-
vations made by him of the great comet of 1860 : ' On the
evening of the 5th of July, whilst I was observing the comet
at sea, I saw a rather intense light from time to time arise in
those portions of the tail that were furthest from the nucleus.
Sometimes instantaneous, and appearing upon a small exten-
sion of the extremity of the tail, which then became more
visible, these fugitive gleams reminded me of the pulsations of
the aurora borealis. At other times they were less fleeting,
and their propagation in rapid succession could be followed for
of a great fire burning in the distance. The comet of 615, observed by the
Chinese, had what appeared to be a movement of libration in its point.
But the analogy of these phenomena with those that we shall describe does not
seem very evident.
306
SUDDEN CHANGES IN COMETAflY LIGHT.
some seconds in the direction of the nucleus near the extremity
of the tail. These appearances then resembled the progressive
undulations of the aurora borealis, but even in this case they
were only visible in the last third of the length of the tail.
The gleams in question were similar to those that I remember
to have seen in the tail of the great comet of 1843, and which
were observed by very many astronomers.'
Are these variations incidental to the comet itself ? It has
been doubted : it has been supposed that they are produced
by sudden changes in the transparency of our atmosphere.
Olbers has made the objection that, if a real and instan-
taneous change had taken place in the brightness of the tail, it
could not have been seen from the earth in so short a time as
a few seconds, as, the different parts of a tail several millions
of miles in length being situated at very unequal distances
from the earth, the times of transmission of the light from
each extremity to the observer would not be identical, and
hence an interval of some minutes would be required to pro-
duce the appearance of the propagation of a luminous change
from one end of the tail to the other. Now observers speak of
variations much more rapid — of some seconds, in fact. M.
Liais reduces this objection to its just value by pointing out
that long cometary tails generally ' front ' us and are not seen
as it were sideways, so that the difference of distance between
the earth and each extremity of the tail is not so great as
Olbers had supposed. For example, * the difference of time
occupied by the light in coming from the two extremities of the
tail of the comet of 1860 to the earth did not amount to four
seconds on the 5th of July.' The same observer likewise remarks
that' the undulations seen by him took place only in a portion
of the tail, and that on the same evening he made compa-
rative observations of the Milky Way and the zodiacal light, but
without being able to detect in either luminous movements
307 x 2
similar to those exhibited by the cometary light. It appears
clear, therefore, that the phenomenon was not occasioned by
variations in the transparency of our atmosphere. It will be
necessary, therefore, to seek the true explanation in the comet
itself, in the actual variations of its light either in the nucleus
or in the tail.
308
SECTION V.
DO COMETS SHINE BY THEIR OWN OB BY REFLECTED LIGHT ?
Do the nuclei of comets exhibit phases ? — Polarisation of cometary light — Experi-
ments of Arago and of several contemporary astronomers — The light of nebulo-
sities and atmospheres is partly light reflected from the sun.
IN the last century astronomers were almost entirely preoccu-
pied with the study of cometary movements, the nature of
cometary orbits, the periodicity of comets, and with every
question, in fact, that tended to prove that, like the planets,
these bodies are subjected to the universal law of gravitation.
Astronomical physics was then hardly recognised, and conjec-
ture filled the place of modern analytical research. It was
doubtless owing to this preoccupation that comets were at
that time looked upon as bodies of kindred nature to the
planets. There was a kind of reaction against the ancient
hypothesis of terrestrial meteors and transient fires. ( Planets
are opaque bodies,' says Pingre; 'they only send back the
light which they receive from the sun. We ought not, per-
haps, to conclude definitively that comets are also opaque
bodies ; it is not absolutely proved that a luminous body
may not circulate around some other body. But the light
of comets is feeble and dull; its intensity varies; we can
perceive in it sensible inequalities and even gaps. It does
not appear that these phenomena can be explained otherwise
than by supposing comets to be opaque bodies, possessed of no
309
THE WORLD OF COMETS.
other light than that which they receive from the sun, and
surrounded by an. atmosphere similar to that of the earth.
Clouds are formed within this atmosphere, just as in our own
atmosphere; these clouds weaken or totally intercept the rays of
the sun, and successively deprive us of the sight of a portion
of the comet. This hypothesis would explain everything. . .'
The same author says elsewhere : ' The nucleus or the head of
a comet is the most brilliant and at the same time the smallest
part of it, and is supposed, with reason, to be a solid body of no
great size, and probably of small density.' This, the reader
will see, is mere conjecture.
Other savants have assumed that comets are planets of a
particular kind, and do not receive their light from the sun,
but shine by their own brilliancy ; but no observations or
proofs have been given in support of this opinion.
We must, however, remark that certain comets have been
thought to exhibit phases. Cassini, when observing the comet
of Cheseaux, in 1744, 'noticed the phase of that comet, the
illuminated portion of which was only half- visible.' These
last words are Lalande's ; Cassini himself only mentions the
irregularity of the nucleus of the comet. In the year 813 'a
comet appeared which resembled two moons joined together ;
they separated, and after taking different forms resembled at
last a man without a head.' Pingre explains this singular
appearance by the phases of the nucleus and tail, the comet
being then near its conjunction with the sun. More precise
testimony is afforded by an observer who saw, first as a
crescent, and then in first quarter, the nucleus of the comet
of 1769 when it was approaching the sun. Arago has discussed
the observations made at Palermo in 1819 by Cacciatore,
and which had induced that astronomer to believe that the
comet in question had exhibited phases. Arago bases his re-
futation upon a drawing made by Cacciatore on July 5, 1819
310
DO COMETS SHINE BY THEIR OWN LIGHT?
(fig. 51), in which the cusps of the crescent are situated in a
line directed towards the sun, instead of at right angles to it,
as they were ten days later, on July 15.
The absence of phases in cometary nuclei is not an argu-
ment against their opacity. Pingre remarked: 'If comets are
true planets, either their heads or their nuclei must of neces-
sity be opaque bodies, illuminated by the rays of the sun;
but these rays also penetrate the atmospheres, which often
send on to us even more light than the body of the comet.
Fig. 51. — Supposed phases of the Comet of 1819, according to Cacciatore:
observations of July o and 15.
It is doubtless for this reason that comets are not seen crescent-
shaped or in quadrature, as is the case with the moon, Mercury,
and Venus.' The same reason has been given by Arago in other
terms. 1 1 confess,' he observes, ' that the absence of phases
in a nucleus, perhaps diaphanous, and surrounded, as is that
of a comet, by a thick atmosphere, which, by reflexion, is
able to distribute light in all directions, cannot lead us to any
certain conclusions.'
Lastly, let me here add a remark which it is hardly possible
anyone could fail to make, on comparing together the telescopic
views of certain comets, as, for example, those of the head of
311
THE WORLD OF COMETS.
Donati's comet. The luminous sectors issuing from the head
of the comet might easily be mistaken for phases in instru-
ments of insufficient power, whilst it is evident that these
variable phenomena, the succession and oscillations of which
are so remarkable, are of quite a different nature, and are not
simple optical appearances.
The problem involved in the nature and constitution of
cometary light was at length attacked by Arago in a new
direction, and by a method which enables the observer to
determine whether the light of an observed comet is that of
matter luminous in itself, or whether it is, wholly or in part,
solar reflected light. This method has been applied to the
nuclei, we shall see further on, as well as to the atmospheres
and to the light of cometary tails. The first researches of Arago
on this subject date from 1819 ; eleven years after Malus had
discovered polarisation by reflexion, and eight years after
Arago himself had remarked the phenomena of the colours of
polarised light.
This is not the place to explain how the nature of any
source of light may be studied by the aid of an optical appa-
ratus called a polariscope. We shall only state that when a
luminous object is examined by the aid of a Nicol's prism or a
thin plate of tourmaline, two images are formed, which vary
in intensity and colour as the apparatus is turned com-
pletely round through four right angles, if the light emitted
from the object is polarised. But if, on the contrary, the light
is natural, the images manifest neither difference in intensity
nor difference in shade of colour. And further, when the
light is polarised we can determine whether it has been polar-
ised by reflexion, and if so, in what plane, so that we can
thus obtain information about the source from which it was
emitted.
Applying these principles to the study of comets, Arago
312
DO COMETS SHINE BY THEIR OWN LIGHT?
subjected to examination the light of the comet of 1819, and
afterwards that of Halley's cornet, in 1835. 'I directed,' he
remarks, 'upon the comet (that of 1819) a small telescope
furnished with a double refracting prism : the two images of
the tail of the comet presented a slight difference of intensity,
which was verified by the concordant observations of Hum-
boldt, Bouvard and Mathieu. On the 23rd of October, 1835,
having applied my new apparatus (the telescope-polariscope)
to the observation of H alley's comet, I saw immediately two
images exhibiting complementary tints, the one red, the other
green. On turning the telescope through 180° the red image
became green, and the green became red. The light of the
comet, therefore, was not composed of rays having the proper-
ties of direct light ; it was reflected or polarised ; that is to
say, definitely, it was light that had proceeded from the sun.'
But might not the results be due to the terrestrial atmo-
sphere? To be assured on this point, Arago directed the same
telescope at the time of his first observation upon Capella, and
found the two images of the star perfectly equal in intensity.
The light of the atmosphere not being polarised, it became
evident that the polarisation was effected at the surface of the
cometary matter.
These observations have since been confirmed by numerous
savants. Chacornac, at Paris ; Ronzoni and Govi, in Italy ;
Poey, at Havana; and Liais, in Brazil, have found that the light
of Donati's comet was polarised either in the nucleus or in the
part of the tail adjacent to the nucleus. The condition which
Brewster insisted upon as essential to the removal of all doubt
in regard to the possibility of polarisation by refraction in the
terrestrial atmosphere has been fulfilled, for M. Poey found
that the plane of polarisation passed through the sun, the
comet, and the eye of the observer ; so that some portion, at
least, of the light of the comet was reflected solar light.
313
THE WORLD OF COMETS.
But to what extent is this the case ? In addition to the
light reflected from the sun have comets no light of their
own ? It is for spectral analysis to reply. We are about to
see if this method of observation,, which as yet has been
applied only to a few not remarkable comets, may not afford
some information on the subject. Let us beforehand,
however, call attention to two interesting observations made
in 1861 and 1868 by Father Secchi. The first relates to
the great double-tailed comet of 1861. At first the nucleus
presented no trace of polarisation, whilst the light of the tail
was strongly polarised. On July 3 the nucleus gave traces of
polarisation. Father Secchi concluded that, during the first
few days, the nucleus shone by its own light — ' perhaps,' he
observes, ' on account of the incandescent state to which the
comet had been brought by its close proximity to the sun.'
The second observation was made in 1868, and refers to Win-
necke's comet. Having examined its light by the aid of a
telescopic polariscope, the same observer found no appreciable
difference of colour in the images of the nucleus ; whilst the
light of the aureola about the comet exhibited an evident
trace of complementary colour. ' Thus,' he concludes, ( the
light of the nucleus is principally its own.' We shall presently
see that further observations of the same kind have been made
recently on Coggia's comet of 1874.
314
SECTION VI.
SPECTRAL ANALYSIS OF THE LIGHT OF COMKTS.
[Researches of Huggine, Secchi, Wolf, and Eayet — Spectra of different comets : bright
bands upon a continuous luminous ground — Analysis of the light of Coggia's comet
in 1874 — Chemical composition of different nuclei and nebulosities.
PHYSICISTS, it is well known, recognise three orders of spectra
as produced by sources of light when a luminous beam ema-
nating from these sources has been decomposed in its passage
through a prism or a system of prisms.
A spectrum of the first order consists of a continuous
coloured strip, exhibiting neither dark lines, nor bright bands
separated by dark intervals ; it is, in fact, the solar spectrum,
more or less brilliant in colour, and of more or less extent, but
destitute of the fine black lines which belong to the spectrum
of the sun. Incandescent solids or liquids produce these
continuous spectra. Spectra of the second order are those
which arise from sources of light composed of vapours or in-
candescent gas; they consist of a greater or less number of lines
or brilliantly coloured bands, separated by dark intervals; the
number, the position, and consequently the colours of these lines
or luminous bands are characteristic of the gaseous substance
under ignition. Every chemically simple body, every compound
body which has become luminous without decomposition, has
a spectrum peculiar to itself. By the inspection of the bril-
liant lines furnished by a gas or incandescent vapour we can
315
THE WORLD OF COMETS.
discover the chemical elements of which it is composed.
Lastly, a spectrum of the third order is one which, like the
spectrum of solar light, may be regarded as formed of a con-
tinuous spectrum intersected by black lines, more or less fine,
but generally much narrower than the luminous intervals be-
tween them. These dark lines indicate the existence of
absorbent vapours in front of the source of light which pro-
duces the continuous spectrum. Wherever a black line is found
to exist, the luminous wave, the refrangibility . of which is
determined by the position of the line, has become extinct.
Experiment has shown that the substances of which these
vapours are formed have the property of intercepting luminous
rays of the same refrangibility as those which they them-
selves emit in an incandescent state. Incandescent sodium,
for example, gives a spectrum of one luminous line, situated in
the yellow portion of the spectrum; on the other hand, solid
incandescent carbon gives a continuous spectrum ; but if the
vapour of sodium surround the carbon, the spectrum will show
a black line in place of the yellow sodium line. A spectrum
of the third order, therefore, indicates a light emanating from
a solid or liquid body, itself surrounded by an atmosphere of
absorbent vapours.
Having stated these elementary facts, let us now see what
results have been obtained by the application of the prism to
the analysis of cometary light.
The spectrum of the comet of 1864 was found by Donati
to consist of three brilliant lines. It is the first observation of
the kind with which we are acquainted. ' The spectrum of this
comet,' he observes, ' resembles the spectra of metals : the
dark portions are broader than those that are more luminous ;
it may, therefore, be considered as a spectrum formed of three
brilliant lines.' This simple observation contains nearly all
that spectral analysis has made known in its application to the
310
SPECTRAL ANALYSIS OF THE LIGHT OF COMETS.
light of comets. The spectrum consisting of three bright lines
or luminous bands has been found, up to the present time, in
every comet that has been analysed ; only the refrangibility
of these bands appears to vary with different comets, indi-
cating either a difference of physical condition or a difference
otherwise but little apparent, in their chemical constitution.
There are other peculiarities, however, which deserve mention,
and these we will successively call attention to.
The comet of 1866, 1., discovered by Tempel, was analysed
both by Mr. Huggins and by Father Secchi. * The light which
emanated from the nucleus,' says the first observer, ' was that
of a broad continuous spectrum fading away gradually at both
edges. These fainter parts of the spectrum corresponded to
the more diffused marginal portion of the comet. Nearly in
the middle of this broad and faint spectrum, and in a position
in the spectrum about midway between b and F of the solar
spectrum, a bright point was seen. The absence of breadth of
this bright point in a direction at right angles to that of the
dispersion showed that this monochromatic light was emitted
from an object possessing no sensible magnitude in the tele-
scope. This observation gives us the information that the
light of the coma of this comet is different from that of the
minute nucleus. The .nucleus is self-luminous, and the matter
of which it consists is in the state of ignited gas. As we
cannot suppose the coma to consist of incandescent solid
matter, the continuous spectrum of its light indicates that it
shines by reflected solar light.
4 Since the spectrum of the light of the coma is unlike that
which characterises the light emitted by the nucleus, it is
evident that the nucleus is not the source of the light by which
the coma is rendered visible to us. It does not seem probable
that the matter in the state of extreme tenuity and diffusion in
what we know the material of the comae and tails of comets to
317
THE WORLD OF COMETS.
be could retain the degree of heat necessary for the incan-
descence of solid or liquid matter within them. We must
conclude, therefore, that the coma of this comet reflects light
received from without; and the only available foreign source
of light is the sun.'
o
In the lio-ht of the same cornet the Roman astronomer
distinguished three lines, one of which— the middle of the three,
of moderate brightness, and possibly that which was seen by
Mr. Huggins — was situated in the green portion of the con-
tinuous spectrum, between the lines b and F of Fraunhofer;
the t\vo others, which were very faint, were situated, the one
in the red, the other towards the violet. Beyond these lines
appeared matter slightly diffused.
The cornet of 1867, II., gave a spectrum probably analogous,
but less distinct. ' In the spectroscope,' says Mr. Huggins,
' the light of the coma formed a continuous spectrum. I was
unable, on account of the faintness of the nucleus, to distinguish
with certainty the spectrum of its light which was projected
upon the large spectrum of the coma. Once or twice I sus-
pected the presence of two or three bright lines, but I could
not be certain on this point.'
The cornet of 1868, I. (Brorsen's), exhibited to the same
observer a spectrum of three brilliant bands projected upon a
faint continuous spectrum. ' The middle band,' says Mr. Hug-
gins, ' is so much brighter than the others that it may be con-
sidered to represent three-fourths, or nearly so, of the whole
of the light which we receive from the comet... In this ne-
bulous band, however, I detected occasionally two bright lines,
which appeared to be shorter than the band, and may be due
to the nucleus itself.' Father Secchi, at Rome, analysed
the light of the same comet, the spectrum of which likewise
appeared to him discontinuous, and formed of luminous bands,
upon a ground slightly luminous. The brightest of these
318
SPECTRAL ANALYSIS OF THE LIGHT OF COMETS.
bands was situated in the green, near to the magnesium
line b. Another was visible in the blue, beyond the line
E, but was less vivid and more vaporous. Two other
lines were seen as well, the one in the yellow, the other,
which was hardly perceptible, in the red. This makes
in all four bands, instead of the
three seen by Mr. Huggins ; but
the faintness of one of them per-
fectly explains this difference in
the results of the two observa-
tions.
Three luminous bands likewise
formed the spectrum of the light of
the comet of 1868, II. (Winnecke's),
4 The middle one,' says Father
Secchi, ' which is the brightest, is
in the green; another, moderately
brilliant, is situated in the yellow ;
and the last and faintest in the blue.
The field of the telescope is full of
a faint diffused light/ The posi-
tions of the luminous bands were
measured by M. Wolf, who found
that the most brilliant was situated
between b and F of Fraunhofer,
nearly in contact with b. Of the two others, the one was
situated between D and E, a little nearer E than D ; the
third beyond F, but close to it. In fig. 53 are shown the two
spectra of the comets of Brorsen and Winnecke, compared with
the spectra obtained from an induction spark in olive-oil and
in a current of olefiant gas. This interesting comparison is
due to Mr. Huggins. There is a very close accordance between
the spectrum of olefiant gas (C2H4) and that of Winnecke' s
319
Fig. 52.— Comet of 1868, II. (Win-
necke's). From a drawing made
by Mr. Huggins.
THE WORLD OF COMETS.
comet, whilst the spectrum of Brorsen's comet is notably dif-
ferent, if not in its composition, at least in the fact of its bands
being situated nearer together.
Comet I. of 1870, observed by Messrs. Wolf and Rayet,
gave three brilliant bands similar to the preceding, projected
upon a faint continuous spectrum. According to Mr. Huggins
the same result was aiforded by the comet of 1871, I. and
I
moo sao
I I L
Fig. 53.— Spectra of the light of the comets of 1868, I. (Brorsen), and 1868, II. (Winnecke)
from the observations of Mr. Huggins : (1) Solar spectrum ; (2) spectrum of carbon spark
taken in olive-oil ; (3) spectrum of carbon spark taken in olefiant gas ; (4) spectrum
of comet of 1868, II.; (5) spectrum of Brorsen's comet, 1868, I.; (6) spectrum of an
induction spark.
Encke's comet, with the difference, that the spectrum of the
latter was not continuous ; this Mr. Huggins attributes to the
small size'and slight brilliancy of the nucleus.
Two other comets were in like manner analysed by Messrs.
W olf and Rayet with the following result: —
320
SPECTRAL ANALYSIS OF TIIE LIGHT OF COMETS.
1 The comet discovered at Marseilles by M. Borrelly,' they
observe, ' on the night of the 20th-21st of August (1873,
III.), presents the form of a circular nebulosity, about two
minutes in diameter, provided with a tolerably brilliant nucleus
in the centre. The spectrum is composed of a continuous
spectrum extending from the yellow nearly to the violet, due
in part to the solar reflected light, and of two luminous bands,
the one in the green, the other in the blue. The green band
is intense, clearly defined towards the red, but diffused to-
wards the violet. The continuous spectrum is much brighter
than that which we have observed in preceding comets, and
much narrower. Perhaps this is due to a solid nucleus.'
In the same year was discovered at the Observatory at Paris,
by MM. Paul and Prosper Henry, two young astronomers, a
comet (1873, IV.), the light of which, when analysed on two
occasions, gave the spectrum represented in fig. 54. On the
Fig. 54.— Spectrum of the Comet 1873, IV. (Henry's) (1) August 26; (2) August 29.
nights of the 26th-27th of August the comet exhibited the
form of a circular nebulosity with a very bright condensation of
light at its centre ; its appearance was that of the stellar mass
in Hercules when seen through an instrument of insufficient
optical power to resolve it into stars.* The spectrum was
* Fig. 32 (p. 217) represents Henry's comet, as seen in the telescope at the
date of these observations.
321 Y
THE WORLD OF COMETS.
composed of the three usual luminous bands, with this peculi-
arity, that the most brilliant line, that in the green, was twice as
lon<>- as either of the other two. There was no trace of a con-
tinuous spectrum. On the night of the 29th-30th the comet
had a tail 25' long, and its central nucleus had increased in
Fig. 5.3. — Coggia's comet, June 10, 1874, according to the drawing of M. G. Eayet.
brightness from the seventh to the sixth magnitude. The
head of the comet always gave a spectrum composed of three
luminous bands, but traversed this time by a very faint con-
tinuous spectrum. The brightness of the comet having in-
322
SPECTRAL ANALYSIS OF THE LIGHT OF COMETS.
creased, we were enabled to make the spectral observations
with a comparatively narrow slit, and the band in the green
then became more distinctly visible. In one portion of its
length it was bounded on both sides by straight lines, but was
throughout more brilliant on the side of the red. The bril-
liancy of the red and blue lines had also increased a little.
Since writing the above lines five new comets have been
discovered and observed in the first six months of the year
1874 : the first on February 20, by M. Winnecke; the second
on April 11, by MM. Winnecke and Tempel; the third, and
the most brilliant, which was visible in July to the naked eye,
was discovered by M. Coggia, at Marseilles, on April 17. The
other two were discovered, the one by M. Borrelly, the other
by M. Coggia. But it is to the spectra of the second and
third that the following results refer : —
' On the morning of the 20th of April/ says Father Secchi,
1 the light of the comet (the second) was moderately bright ;
it exhibited a nucleus surrounded by an irregular fan-shaped
nebulosity. The simple spectroscope applied to the great tele-
scope of Merz showed traces of bands, but the diffusion of
the object did not permit the use of this instrument. The
compound spectroscope was applied, the telescope being also
used, for so faint was the object that nothing could be made
out with certainty. Then, on removing the telescope and
looking with the unassisted eye, the spectrum appeared very
clearly formed of three distinct bands, well separated: one in
the blue-green, another in the green, and the third in the yel-
low-green. The first was the most brilliant and most extended.
My impression is that these bands occupied the same places as
the bands of the other comets, but I was unable to make exact
measurements.'
The same astronomer thus describes his two spectroscopic
observations of Coggia's comet: 4 On the 17th of May I was
323 Y 2
THE WORLD OF COMETS,
able to ascertain that the spectrum consisted of bands; two
especially were very bright in the green and the yellow-green.
Having illuminated the tube of the telescope in front of the
slit with the diffused light of various gases, the two bright
bands were found to correspond with the bands of carbonic
oxide and carbonic acid. The faintness of the light did not
allow of the recognition of the other bands.'
The light of the same comet was subjected to analysis by
MM. Wolf and Rayet. 'On the 19th of May/ observes the
latter, ' I was able, in conjunction with M. Wolf, to make a
first spectroscopic observation with some completeness. The
diameter of the comet was nearly three minutes, and a tail was
beginning to develop itself. The light, when analysed by the
prism, gave a continuous spectrum from the orange to the blue
(the spectrum of the solid nucleus), crossed by three bright
bands (the spectrum of the gaseous nebulosity). It was the
well-known cometary spectrum, but it differed from the ordinary
spectrum in the dimensions and relative brilliancy of its
different parts. Thus, whilst the continuous spectrum of the
nucleus is in general wide and diffused, the spectrum given
by Coggia's comet was very narrow. And again, the luminous
transverse bands, instead of being ill-defined towards the most
refrangible side, were terminated both towards the red and
violet by straight and tolerably sharp lines. The remarkable
fact of the central band being the longest and the most luminous
struck me forcibly, as I had never witnessed it before.'
This last-mentioned fact was confirmed by a second obser-
vation, made on the night of June 4th-5th. ' The continuous
spectrum,' says M. Rayet, ' corresponding to the nucleus, is
remarkably narrow — nearly as narrow as that of a star seen
through the same instrument ; it is not unlike the spectrum
of a star of the sixth magnitude, but it is colourless towards
the extremities. The spectrum extends on both sides beyond
324
SPECTRAL ANALYSIS OF THE LIGHT OF COMETS.
the luminous bands. The spectrum of bands is composed of
three lines, which by their refrangibility correspond to the
yellow, the green, and the blue. The central band is long and
very luminous ; and when the aperture of the slit is suitably
diminished it is terminated, towards the red and violet, by
sharply-defined lines ; it shows, therefore, none of that fadirig-
off appearance towards the violet which is found in the spectra
of ordinary telescopic comets. . . The bands in the yellow
and blue are about half as bright as the middle one; they are
slightly diffused towards the edges, and approximate to the
ordinary type.
4 If, instead of directing the slit of the spectroscope upon the
focal image of the nucleus, so as to obtain at once the spectrum
of the nucleus and that of the nebulosity, the slit is so turned
as to cut the image of the tail, a spectrum is then obtained
which presents the three bright bands above described, without
a trace of the continuous spectrum, and separated from each
other by dark intervals. In the tail, therefore, there is no solid
incandescent matter of sensible amount.'
In the next section of this chapter other details will be
found regarding the analysis of the light of the comet of
1874. They were received too late to be inserted in this sec-
tion to which they naturally belong. These details, we may
remark, confirm the results of MM. Wolf and Rayet.
Such are the results that have been afforded up to the
present time by the spectral analysis of light. They are im-
portant on account of the conclusions we may even now permit
ourselves to draw from them respecting the physical and
chemical constitution of several cometary bodies.
In the first place, there is one fact common to all comets
whose light has been analysed — the fact that their spectrum
principally consists of a certain number of light bands separated
by dark intervals of some extent. The continuous and very
325
THE WORLD OF COMETS.
faint spectrum upon which these bands are projected existed,
or at least was visible, only in some cases. Comets whose nuclei
are very faint, like that of Encke's comet, or not sufficiently
luminous (comet 1873, IV.), have failed to give a continuous
spectrum. We may consider, therefore, that the bright bands
are not produced by the light of cometary atmospheres or
coma3. From his first observations Mr. Hnggins came to an
opposite conclusion, but this was doubtless owing to the im-
possibility of comparing the results then obtained with those
afforded by the comets which have been analysed since.
We may thus regard the comets with nuclei which have
been analysed by the spectroscope as constituted as follows : —
In the centre of the nebulosity a nucleus giving a con-
tinuous spectrum. Does this necessarily imply a liquid or solid
incandescent matter ? We might answer in the affirmative if
the continuity of the spectrum could be regarded as complete ;
but it is so faint that it is difficult to say with certainty
whether the light with which it shines really belongs to the
incandescent matter of which it is composed, or if it is light
reflected from the sun. It is not improbable that this light is
of both kinds, especially when the comet is drawing near the
sun and is subjected to a continually increasing temperature.
The observations of polarisation by reflection prove that in any
case a part of the light is reflected from the sun.
As regards the light of the atmospheres and tails, the
spectrum of bright bands denotes alike the gaseous and the
incandescent state of the matter of which they are composed.
The identity in this respect of the tail and coma of Coggia's
comet clearly shows that it is the matter of the atmosphere
which, under the influence of a repulsive action, helps to form
the cometary appendage opposite the sun. As, on the other
hand, the phenomena of sectors emanating from the nucleus
prove that the atmospheric envelopes are formed at the expense
326
SPECTRAL ANALYSIS OF THE LIGHT OF COMETS.
of the nucleus, it is very difficult to admit the incandescent state
of the cometary atmosphere and tail without admitting that the
nucleus, the seat of their continual formation, is likewise in an
incandescent state. It is, then, probable that the nucleus, at all
events in the vicinity of the perihelion, emits, besides light
reflected from the sun, direct light that has emanated from
its own substance.
In a chemical point of view the comets — few in number, it
is true — which have as yet been subjected to examination are of
very simple constitution. They consist of simple carbon, or of
a compound of carbon and hydrogen, according to the com-
parisons made by Mr. Huggins ; carbonic oxide or carbonic
acid, according to the researches of Father Secchi. The
Italian astronomer was, therefore, justified in saying: 'It is
very remarkable that all the comets observed up to the pre-
sent time have the bands of carbon,'
327
SECTION VII.
THE COMET OF 1874, OR COGGIA'S COMET.
Of the five comets of 1874 the third, or comet of Ooggia, was alone visible to the naked
eye Telescopic aspect and spectrum of the comet during the early part of its
apparition, according to Messrs. Wolf and Rayet — Observations of Secchi, Bredi-
chin, Tacchini, and Wright ; polarisation of the light of the nucleus and tail—
Transformations in the head of the comet between the 10th of June and the 14th
of July, according to Messrs. Rayet and Wolf.
THE comets, and not the comet, of 1874 should form the
title, strictly speaking, of the present section of our work.
Indeed, at the time of adding these lines to this chapter —
that is to say, in the last few days of the month of August
of this year [1874] — five new comets have been discovered
and observed. But one only, the third in order of date, has
attracted the attention of the public, for the simple reason
that it alone became bright enough during the time 'of its
apparition to be visible to the naked eye. The other four con-
tinued to remain telescopic comets, accessible only to profes-
sional astronomers. Although its visibility in Europe was of
brief duration, the comet of 1874, III., or comet of Coggia,
presented in its physical aspect, and in the changes of form
in its head and tail, sufficiently curious phenomena to merit
special mention and some detailed description.
At the Observatory of Marseilles, on the night of April
17, the new comet was discovered by an astronomer of
that establishment, M. Coggia, already known in the scien-
328
Pi.. XI.
COGGIA'S COMET, 1874.
SEEN FROM THE PONT-NEUF, PARIS.
THE COMET OF 1874, OR COGGIA/S COMET.
tific world by his discovery of the planet -<Egle, the second
comet of 1870, and last year by his discovery of the seventh
cornet of that year, of which we have already made mention
in the chapter on Periodical Comets. The new comet on
its first appearance was a very faint nebulosity, but as it
advanced nearer to the sun and the earth it grew rapidly
brighter, and became visible to the naked eye about the begin-
ning of July. From this date the comet continued to increase
in brilliancy up to the night of July 14, when its own and
the diurnal movement combined caused it to subside into the
mists of the horizon and finally disappear from our latitude.
It is to be hoped that it will have been observed in regions
nearer the equator and in the southern hemisphere. This is
greatly to be desired, for it disappeared from us at the very
moment when its telescopic study had become of the very
highest interest. We will, therefore, limit ourselves to the facts
observed, letting each observer speak for himself.
' At the date of its discovery,' says M. Rayet, l the comet
was faint and of a circular form, with a very marked central
condensation, resembling a luminous point. The nebulosity was
about two minutes in diameter. The light was of such small
intensity that it was hardly possible to verify the existence of
a spectrum. The comet continued to approach the sun and
the earth, and its brilliancy steadily increased.'
The spectral analysis of its light made jointly by Messrs.
Rayet and Wolf, on May 19, has been given in the
preceding section as well as that made on the night of the
4th-5th of June.
On the night of the 4th-5th of June the comet exhibited
a round and very brilliant nucleus, about equal in brilliancy
to a star of the eighth magnitude. The surrounding nebulosity,
from which the nucleus stood out very distinctly, measured
four minutes in diameter, and was prolonged opposite the sun
329
THE WORLD OF COMETS.
into a tail eight minutes in length. Its light, which had
quadrupled in intensity since April 17, gave a clearly visible
spectrum, which we have already described on page 324
according to M. Rayet.
The light of the comet was likewise analysed at Rome by
Father Secchi, whose observations confirm in their principal
results those of the French savants. The three bright bands
and the continuous spectrum which crossed them transversely
presented, on the dates of June 18 and July 9, the ap-
pearance shown in fig. 56. Father Secchi directs attention
to a peculiarity which is readily to be perceived, viz. the
discontinuity in the continuous spectrum in the neighbour-
hood of each band, and which is more especially apparent in
the second observation. ' On examining,' he observes, ' the
spectrum thus composed with a Kicol's prism, the contin-
uous portions were diminished in intensity, whilst the bands
themselves lost none of their brilliancy. This observation
would lead us to believe that the continuous spectrum was
derived from reflected light.' We see that in this respect
Father Secchi differs in opinion from the French astronomers
quoted above, who consider the continuous spectrum as pro-
duced by a solid nucleus in a certain state of incandescence.
It appears certain that the light of the comet was polarised; this
was proved by the observations made at Rome in the course
of July ; but may not the nucleus at the same time both emit
its own light and reflect that of the sun ? This is a question
not yet solved, and doubt still exists concerning the nature of
cometary light.
We add further details, due to the same astronomer, who
was enabled to observe the comet for a longer time than was
possible in France, as is shown by the date of July 17,
mentioned below.
' The comet,' he remarks, ' when observed with an ordinary
330
THE COMET OF 1874, OR COQGIA'S COMET.
eyepiece was magnificent. On the 9th of July it formed a fan
of a reddish tint (by contrast with the nucleus) of about 180
degrees of opening, composed of curvilinear rays springing from
a nucleus of yellowish green. On increasing the magnifying
power to 100 the nucleus was seen surmounted only by very
faint plumes and reduced in size to a small diffused sphere
hardly two seconds in diameter. The absence of all defined
limit, an effect produced by the high magnifying power em-
ployed, proves that no solid body was contained in the nucleus.
The same power shows, in fact, the satellites of Jupiter with
clearly-defined discs.
Fig. 56. — Spectra of the Comet of 1874, III., (Coggia's), according to Father Secchi.
1 At the request of Mr. Hind we have looked for the comet
during the daytime, but without success. There appeared little
probability of seeing it under these conditions, for Jupiter, a
much more brilliant object, was not visible. On the 17th of
July the tail was enormous ; it extended to the star u of the
Great Bear, the head being hidden below the horizon. It must
have been at least 45 degrees in length. On the 13th it was
very expanded near the head.'
On comparing the position of the bright bands of the
331
THE WORLD OF COMETS.
cometary spectrum with the spectra given by carbon and car-
bonic acid, Father Secchi found them to correspond ; but, on
employing hydrocarbons, no hydrogen line appeared to coincide
with those of the comet. These results show that astronomers
are not yet well agreed in their interpretation of the facts
afforded by spectral analysis, for we read in a letter addressed
from Moscow to the Italian Spectroscopical Society by Pro-
fessor Bredichin, that this savant compared the positions of
the bands of the comet with those of a hydrocarbon in a
Geissler's tube ; and he adds : ' Within the limits of the errors
of the observations (I made ten) the bands of the comet coin-
cide with the bands of the hydrocarbon whose wave-lengths
are 5633, 5164, 4742 of Angstrom's scale'
At Palermo, M. Tacchini has also made the following ob-
servations upon the spectrum of the comet and the polari-
sation of its light : —
' The bright lines observed in the spectrum of the comet
were four in number, corresponding, when referred to the
solar spectrum, to the following positions of Angstrom's scale :
6770, 5620, 5110, and 4800. The position of these lines can-
not be looked upon as strictly accurate, on account of the
manner in which they were obtained ; but it is evident that
the three last correspond to the spectrum of carbon. The red
line was less distinct than the others, because in this part the
red was bright and diffused. This line was only well seen in the
last days of June and the first days of July. The three other
lines were not of equal length, and the longest was the 5620
line ; the 5110 line was the brightest of all, and appeared almost
as white as the magnesium line after solar eruptions. The con-
tinuous spectrum of the comet's nucleus was projected seem-
ingly upon a ground formed of a more intense solar spectrum,
in which the red was, as has just been said, the most extended.
This beautifully coloured band or ribbon was seen only at the
332
THE COMET OF 1874, OR COGGIA'S COMET.
passage of the nucleus, which, observed through a simple
eyepiece, appeared of a greenish white, whilst the fan was
sensibly rose-coloured, even when occulting the nucleus. In
the bright solar light reflected by the nucleus, traces of polar-
ised light were to be expected ; and to test their existence we
invited Signor Pisati, professor of physics, to make experiments
by the aid of the polariscopes at his disposal. On applying a
bi-quartz to the telescope, traces of polarisation were observed,
but they were very feeble. By the aid of a Nicol's prism the
light appeared strongly polarised, and the greatest diminution
of light took place when the principal section of the prism
was coincident with the direction of the tail, from which
it follows that the light was polarised in a plane passing
through the sun. The experiment was again repeated upon
the brightest portion of the tail, and with the same result.
Towards the middle of the tail the light was so feeble that
nothing certain could be determined; but it seems probable
that reflexion took place throughout the entire length of
the tail.' (Memorie della Societa degli Spettroscopisti Italiani.
Luglio, 1874.)
These very decisive conclusions respecting the polarisation
of the comet's light derive further confirmation from the
observations of Mr. Wright, at Yale College (U.S.), which led
him to infer that a considerable portion of the light of the
comet was derived from the sun by reflexion.
Let us now return to the aspect of the nucleus and
nebulosity, as shown in the telescope during the most interest-
ing period of the comet's apparition. In order to follow the
various appearances presented we shall avail ourselves of the
detailed and very careful descriptions placed at our disposal
by MM. Wolf and Rayet. Thanks to the courtesy of these
gentlemen, we shall be able to study the different changes
exhibited by the comet from original drawings hitherto un-
333
THE WORLD OF COMETS.
published, and which we have received permission from MM.
Wolf and Rayet to engrave.
' On the 10th of June,' they remark, ' the comet preserved
unchanged the same general aspect as on the preceding days; it
was still a circular nebulosity about four minutes in diameter,
and provided with a central nucleus very brilliant and remark-
ably distinct, which gave to the comet a remarkable ap-
pearance. In a direction opposite to the sun the nebulosity
was lengthened out, and thus formed a tail, which, narrow at
its base, expanded into a fan about twenty-four minutes in
length. The coma was more brilliant in the centre than
towards the edges. (See fig. 55, p. 322.)
' The comet preserved the same appearance, whilst increas-
ing rapidly in size, till about June 22, at least so far as it was
possible to judge from observations much impeded by the light
of the moon.
4 The spectrum remained as above described, viz. it con-
sisted of a very narrow continuous spectrum, and of three
bright transverse bands.
' On the 22nd of June a series of changes in the head of
the comet began. On this day the comet, when examined with
the Foucault's telescope, 15| inches aperture, appeared to be en-
closed in the interior of a very elongated parabola. Starting
from the nucleus, situated where the focus of the parabola
would be, the light diminished regularly towards the vertex;
but towards the interior of the parabola the diminution of the
light was abrupt, and its line of separation was another
parabola, slightly more open than the first, and having for
its vertex the brilliant nucleus itself. (See fig. 57.) The
parabola passing through the nucleus formed, when pro-
longed, the sides of the tail, the edges of which were clearly
denned and were much more brilliant than the inner portion.
The tail had, therefore, the appearance of a luminous envelope,
334
THE COMET OF 1874, OR COGGIA'S COMET,
hollow in the interior. The nucleus continued sharp and
bright.
1 On the 1st of July the general form of the comet remained
unchanged : it still appeared bounded on the outside by an arc
of a parabola. The luminous point, however, had shifted
l''ig. <37- — Coggia's Comet seen in the telescope on June 22, 1874, according to a
drawing by M. G. Hayet.
forward into the interior of the second parabola, and the two
sides of the tail were not symmetrical. (Fig. 58.) The west
side (the side on which the right ascension is the greater)
335
THE WORLD OF COMETS.
was sensibly more luminous than the other. The spectrum of
bright bands given by the nebulosity was moderately luminous,
and colours were distinguishable in the narrow spectrum of the
nucleus; the red at one extremity, and a tint of blue or violet
at the other.
Fig. 58. — Coggia's Comet on July 1, 1874, according to a drawing by M. Gr. Kayet.
' Since the 5th of July the comet's want of symmetry has
continued to increase in a marked degree, and towards the
head the diminution of the light has become less regular.
4 On the 7th the want of symmetry was striking, the west
336
THE COMET OF 1874, OR COGGIA'S COMET.
portion of the tail being about twice as brilliant as the east
portion. At the same time the nucleus appeared to have
become diffused and " fuzzy " towards the head, whilst it was
still clear and distinct towards the tail. It suggested the idea
of an open fan.
Fig. 59.— Coggia's Comet on July 13, 1874, according to M. (*. Kayet.
1 From the 7th to the 13th of July the weather was un-
favourable for observations; but in the interval the comet
had undergone no material change, for on the 13th it was
visible again, having the same form, but somewhat more
337 Z
THE WORLD OF COMETS.
pronounced. The fan of light, however, formed at the expense
of the nucleus, had assumed greater importance, and was inclined
in a very marked manner towards the western portion of the
coma. At the moment of observation (fig. 59), about 10 P.M.,
the northern portion of the sky was slightly foggy, and the
Fig. 60. — Coggia's Comet on July 14, according to M. G. Bayet.
comet already close to the horizon. As for the tail, it ex-
tended nearly to o of the Great Bear, and thus had an appa-
rent length of about 15 degrees.
1 Our last observation of the comet was on the 14th, at
388
THE COMET OF 1874, OR COGGIA'S COMET.
9h 30ra P.M. Important changes had taken place in the aspect of
the head. (Fig. 60.) The fan of light was altogether thrown
towards the west, and on this side was prolonged into a lono-
train, losing itself far into the coma. Towards the east the
fan terminated abruptly, and the line of termination made only
a small angle with the axis of the comet. Two plumes or jets
were visible, projecting forward, one on the right, the other
on the left. These plumes seemed to rise from the edge of the
tail, of which they formed as it were the prolongation. The
eastern plume soon curved back towards the tail ; it was faint,
and was soon lost in the nebulosity. The plume directed
towards the west was much more brilliant and curved back
immediately towards the tail, the bright outer edge of which
it helped to define.'
MM. Wolf and Rayet call attention to the fact that the
comets of 1858 and 1861 exhibited transformations similar to
those of the comet of Coggia. The analogy is evident, but
at the same time there are marked differences. The aspect of
the comet of 1874, on the night of July 14, was especially re-
markable for the phenomena indicated in the drawing of M.
Rayet, which we think are unprecedented. The plumes which
have just been described indicate the commencement of a
radical transformation in the form of the head and tail — one
would have said that two different cornets were in juxtaposi-
tion, the one projected upon the other. Was this, as has been
suggested, a premonitory sign of duplication? This is what
we shall learn, if the series of observations unfortunately
interrupted in Europe has been continued in the southern hemi-
sphere. There is, also, a peculiarity which calls for remark
on comparing the drawings of the French astronomers with
those of Mr. Newall, taken nearly at the same time. The two
plumes in the English sketch form two very regular plumes,
symmetrically placed with regard to the axis of the tail, the
339 z 2
THE WORLD OF COMETS.
nucleus and the head of the comet; they irresistibly remind
us of the antennae of certain moths. We must confess that
the very carefully-studied drawing of M. Rayet appears to us
to merit entire confidence.
But let us return to the observations of MM. Wolf
and Rayet. Their concluding remarks relate to the spectral
analysis of the cornet's light during the month of July : —
' Whilst the comet was changing form, its spectrum pre-
served the same character and appearance, and continued to
increase in brightness. It was not until July 13 that it
became modified by the exaggerated importance of one of its
parts. At this time the nucleus had become diffused, and the
solid matter of which it had been composed appeared to be
distributed throughout the head of the comet, so that the spec-
trum consisted of a luminous and vividly coloured streak, con-
tinuous from the red to the violet, standing out from a
continuous and broader spectrum. The three luminous bands
had nearly disappeared, probably drowned in the light of the
continuous spectrum. The comet moreover was situated low
down in the mists near to the horizon. In the continuous
spectrum we looked in vain for the presence of bright lines
or black bands.
' On the 1st and 6th of July, whilst the luminous bands
were yet visible, we referred micrometrically the position of
the most brilliant of them — the middle one — to the lines E and
b. The wave-lengths, on the least refrangible side of the line,
were thus found to be: 1st of July, 5161; 6th of July, 5165.
The wave-length, of the three lines b being 5174, this band
is slightly more refrangible.
' We believe that this measure is accurate ; but the difficulty
of such determinations is so great, that we think it useless to
identify this band with the bright lines of any gas.'
Such are the facts that have been as yet collected con-
.340
THE COMET OF 1874, OR COGGIA'S COMET.
cerning the physical and chemical constitution of the comet
discovered by M. Coggia. We shall refrain from drawing any
conclusions from them, as all discussion at the present moment
would be incomplete and consequently premature. The new
comet has certainly been observed by many astronomers both
in Europe and America; and we must wait for the observa-
tions made subsequently to the disappearance of the comet
from our latitudes.
We shall say nothing of the comets I., II., and V. of 1874,
except that they were discovered, the first two by M. Winnecke,
Fig. 61.— Comet of 1618, according to
Hevelius. Multiple nuclei.
Fig. 62.— Comet of 1661, according to
Hevelius. Multiple nuclei.
on February 20 and April 11 respectively; the third by M.
Coggia, on August 20. But comet IV., 1874, discovered by M.
Borrelly (Marseilles), exhibited a very interesting structure,
which gives some reason to believe that the observations of
the comet of 1618 and 1661 by Hevelius are worthy of more
consideration than they have hitherto received by astro-
nomers. The heads of these comets (figs. 61 and 62) were
furnished with multiple nuclei, so that they were formed to all
appearance of an assemblage of little stars. We give the fac-
341
THE WORLD OF COMETS.
simile of the drawings by which Hevelius has endeavoured to
represent their nucleal structure.
The comets 1869, L, 1869, III., and 1871, 1., were in like
manner formed of nebulosities studded with a great number
of luminous points, which gave them the appearance of certain
resolvable nebula. The comet discovered in 1874 by M.
Borrelly belongs evidently to the same class. M. Wolf thus
describes the appearance presented by the fourth comet of the
year : ' The new comet,' he observes, ' discovered at Marseilles
by M. Borrelly presented from the first the appearance of a some«
what faint but nearly resolvable nebula. Upon the whitish
ground of the nebulosity appear a number of little brilliant
points, of which the most conspicuous is excentric and situated
behind and to the north of the centre of figure. This comet
seems, therefore, to belong to a class whose representatives are few
in number, and to which Schiaparelli has called attention, viz.
to the class of comets composed of a mass of little nuclei. On
August 3 the aspect of Borrelly's comet was not unlike that of
the stellar mass in Hercules, but of less extent and brilliancy.
On August 8 the principal excentric nucleus had become
more brilliant, while the nebulosity had increased in extent.'
ON COGGIA'S COMET (III., 1874).
[ADDITION BY THE EDITOR.]
M. Guillemin's book was published at the end of 1874,
before it was possible to compare together and discuss all the
observations and drawings that had been made of the comet.
I therefore propose in this addition to the chapter to give a
brief account of some of the other observations of this comet,
342
THE COMET OF 1874, OR COGGTA'S COMET.
which must be in the remembrance of every one of our
readers.
The following remarkable and interesting letter from Mr. J.
Norman Lockyer appeared in the Times of July 16, 1874. It
was reproduced in Nature for July 23, 1874: —
' Mr. Newall's Observatory, Ferndene, Gateshead.
'I was enabled on Sunday night (12th inst.), by Mr.
Newall's kindness, to spend several hours in examining the
beautiful comet which is now visiting us, by means of his
monster telescope — a refractor of 25-in. aperture, which may
safely be pronounced the finest telescope in the world, or, at
all events, in the Old World.
1 The view of the comet which I obtained utterly exceeded
my expectations, although I confess they were by no means
moderate ; and as some of the points suggested by the observa-
tions are, I think, new, and throw light upon many recorded
facts, I beg a small portion of space in the Times to refer to
them, as it is important that observers have their attention
called to them before the comet leaves us.
4 1 will first deal with the telescopic view of the comet.
Perhaps I can give the best idea of the appearance of the
bright head in Mr. Newall's telescope, with a low power, by
asking the reader to imagine a lady's fan opened out (160°)
until each side is almost a prolongation of the other. An
object resembling this is the first thing that strikes the eye,
and the nucleus, marvellously small and definite, is situated a
little to the left of the pin of the fan — not exactly, that is, at
the point held in the hand. The nucleus is, of course, brighter
than the fan.
' Now, if this comet, outside the circular outline of the fan,
offered indications of other similar concentric circular outlines,
astronomers would have recognised in it a great similarity to
343
THE WORLD OF COMETS.
Donati's beautiful comet of 1858, with its "concentric envelopes."
But it does not do so. The envelopes are there undoubtedly,
but, instead of being concentric, they are excentric, and this is
the point to which I am anxious to draw attention, and, at the
risk of being tedious, I must endeavour to give an idea of the
appearance presented by these excentric envelopes. Still re-
ferring to the fan, imagine a circle to be struck from the left-
hand corner with the right-hand corner as a centre, and make
the arc a little longer than the arc of the fan. Do the same
with the risrht-hand corner.
O
* Then with a gentle curve connect the end of each arc with
a point in the arc of the fan half-way between the centre and
the nearest corner. If these complicated operations have been
properly performed, the reader will have superadded to the fan
two ear-like things, one on each side. Such "ears," as we may
for convenience call them, are to be observed in the comet, and
they at times are but little dimmer than the fan.
' At first it looked as if these ears were the parts of the head
furthest from the nucleus along the comet's axis, but careful
scrutiny revealed, still in advance, a cloudy mass, the outer
surface of which was regularly curved, convex side outwards,
while the contour by the inner surface exactly fitted the outer
outline of the ears and the intervening depression. This mass
is at times so faint as to be invisible, but at other times it is
brighter than all the other details of the comet which remain
to be described, now that I have sketched the groundwork.
These details consist of prolongations of all the curves I have
referred to backwards in the tail.
' Thus, behind the bright nucleus is a region of darkness (a
black fan, with its pin near the pin of the other pendant from it,
and opened out 45° or 60° only will represent this), the left-
hand boundary of which is a continuation of the lower curve of
the right ear. The right-hand boundary is similarly a con-
344
THE COMET OF 1874, OR COGGIA'S COMET.
tinuation of the lower curve of the left ear. Indeed, I may
say generally — not to enter into too minute description in
this place — that all the boundaries of the several different
shells which show themselves, not in the head in front
of the fan, but in the root of the tail behind the nucleus,
are continuous in this way — the boundary of an interior
shell on one side of the axis bends over in the head to form
the boundary of an exterior shell on the other side of the axis.
1 At last, then, I have finished my poor, and, I fear, tiresome
description of the magnificent and truly wonderful sight pre-
sented to me as it was observed, on the whole, during some
hours' close scrutiny under exceptional atmospheric conditions.
4 1 next draw attention to the kind of change observed. To
speak in the most general terms, any great change in one " ear "
was counterbalanced by a change of an opposite character in the
other, so that when one ear thinned or elongated, the other
widened; when one was dim, the other was bright; when one
was more " pricked " than usual, the other at times appeared to
lie more along the curve of the fan and to form part of it.
Another kind of change was in the fan itself, especially in the
regularity of its curved outline and in the manner in which the
straight sides of it were obliterated altogether by light, as it
were, streaming down into the tail.
' The only constant feature in the comet was the exquisitely
soft darkness of the region extending for some little distance
behind the nucleus. Further behind, where the envelopes of the
tail were less marked, the delicate veil which was over even the
darkest portion became less delicate, and all the features were
merged into a mere luminous haze. Here all structure, if it
existed, was non-recognisable, in striking contrast with the
region round and immediately behind the fan.
' Next it has to be borne in mind that the telescopic object is
after all only a section, from which the true figure has to be
built up, and it is when this is attempted that the unique
345
THE WORLD OF COMETS.
character of this comet becomes apparent. There are no jets,
there are no concentric envelopes; but, as I have said, in place
of the latter, excentric envelopes indicated by the ears and their
strange backward carvings, and possibly also by the fan itself.
4 1 prefer rather to lay the facts before observers than to
state the conclusions to be derived from them, but I cannot
help remarking that, supposing the comet to be a meteor-whirl,
the greatest brilliancy is observable where the whirls cut or
appear to cut each other ; where we should have the greatest
number of particles, of whatever nature they may be, in the
line of sight; and not only so, but regions of greatest possible
number of collisions associated with greatest luminosity.
' It would be a comfort if the comet, to partly untie a hard
knot for us, would divide itself as Biela's did. Then, I think,
the whirl idea would be considerably strengthened. I could
not help contemplating the possibility of this when the mean-
ing of the " ears " first forced itself upon my attention.
' The spectroscopic observations which I attempted, after
the telescopic scrutiny, brought into strong relief the littleness
of the planet on which we dwell, for a seven hours' rail journey
from London had sufficed to bring me to a latitude in which
the twilight at midnight was strong enough to show the middle
part of the spectrum of the sky, while to the naked eye the tail
of the comet was not so long as I saw it in London a week
' I had already, in observations in my own observatory,
with my 6j-in. refractor (an instrument smaller than one of Mr.
Newall's four finders !), obtained indications that the blue rays
were singularly deficient in the continuous spectrum of the
nucleus of the comet, and in a communication to Nature I had
suggested that this fact would appear to indicate a low
temperature.
' This conclusion had been strengthened by Sunday night's
346
THE COMET OF 1874, OR COGGIA'S COMET.
observations, and it was the chief point to which I directed my
attention. The reasoning on which such a conclusion is based
is very simple. If a poker be heated, the hotter it gets the
more do the more refrangible — i.e., the blue — rays make their
appearance if its spectrum be examined. The red colour of a
merely red-hot poker and the yellow colour of a candle-flame
are due, the former to an entire, the latter to a partial, absence
of the blue rays. The colour, both of the nucleus and of the
head of the comet, as observed in the telescope, was a distinct
orange-yellow, and this, of course, lends confirmation to the
view expressed above.
' The fan also gave a continuous spectrum but little inferior
in brilliancy to that of the nucleus itself j while over these, and
even the dark space behind the nucleus, were to be seen the
spectrum of bands which indicates the presence of a rare vapour
of some kind, while the continuous spectrum of the nucleus
and fan, less precise in its indications, may be referred either
to the presence of denser vapour, or even of solid particles.
' I found that the mixture of continuous band spectrum in
different parts was very unequal, and further that the contin-
uous spectrum changed its character and position. Over some
regions it was limited almost to the region between the less
refrangible bands.
4 It is more than possible, I think, that the cometary spec-
trum, therefore, is not so simple as it has been supposed to be,
and that the evidence in favour of mixed vapours is not to be
neglected. This, fortunately, is a question on which I think
much light can be thrown by laboratory experiments.
'P.S. — (By Telegraph.) — Wednesday night.— Sunday's
observations are confirmed. The cometary nucleus is now
throwing off an ear-like fan. Ten minutes' exposure of a
photographic plate gave no impression of the comet, while
347
THE WORLD OF COMETS.
two minutes gave results for the faintest of seven stars in the
Great Bear.'
A rough outline sketch of the head and envelopes of
Coggia's comet, as seen in Mr. Ne wall's 2 5 -inch refractor on
the night of July 12, appeared in Nature for July 16, 1874.
On July 8, Mr. G. H. With, at Hereford, observing the
comet with an 8^-inch aperture Newtonian reflector, noticed a
remarkable oscillatory motion of the fan -shaped jet, upon the
nucleus as a centre, and which occurred at intervals of from
three to eight seconds. ' The fan seemed,' says Mr. With, ' to
tilt over from the preceding towards the following side, and
then, for an instant, appeared sharply defined and fibrous in
structure. Suddenly it became nebulous, all appearance of
structure vanished, and the outline became merged in the sur-
rounding matter. At the moment of this change a pulsation
was transmitted from the head through the coma, as though
luminous vapour had been projected from the former into the
latter. These phenomena were observed many times during
the evening, both by myself and a well- trained optical
assistant,' *
Both Mr. With and Mr. Newall also speak of a faint lumi-
nous cloud that preceded the 'head of the comet, i.e. in
front of it, on the opposite side to the tail, and apparently
separate from the comet. The latter also states that the
eifect of motion was conveyed in a remarkable manner by the
flickering of the tail.
Mr. Huggins's paper on the spectrum of Coggia's comet
was read before the Royal Society on January 7, 1875, and is
printed in Proc. Roy. Soc., vol. xxiii., pp. 154-1 59. The follow-
ing are some extracts from his account: —
* R.A.S. Notices, May 187(5.
348
THE COMET OF 1874, OR COGGIA'S COMET.
1 The comet now visible, which was detected by M. Coggia,
April 17, 1874, is the first bright comet to which the spectro-
scope has been applied. The following spectroscopic observa-
tions of this comet were made from July 1 to July 15 :
' When the slit of the spectroscope was placed across the
nucleus and coma, there was seen in the instrument a broad
spectrum, consisting of the three bright bands which were
exhibited by Comet II., 1868, crossed by a linear continuous
spectrum from the light of the nucleus.
1 In the continuous spectrum of the nucleus I was not able
to distinguish with certainty any dark lines of absorption, or any
bright lines other than the three bright bands.
' Besides these spectra, there was also present a faint broad
continuous spectrum between and beyond the bright bands.
' When the slit was moved on to different parts of the coma,
the bright bands and the faint continuous spectrum were
observed to vary in relative intensity.
' When the slit was brought back past the nucleus on to
the commencement of the tail, the gaseous spectrum became
rapidly fainter, until, at a short distance from the nucleus, the
continuous spectrum predominated so strongly that the middle
band only, which is the brightest, could be detected on it.
' We have presented to us, therefore, by the light of the
comet three spectra : —
' 1. The spectrum of bright bands.
' 2. The continuous spectrum of the nucleus.
4 3. The continuous spectrum which accompanies the gaseous
spectrum in the coma, and which represents almost entirely the
light of the tail.'
Mr. Huggins then describes in detail the spectrum of bright
bands and the continuous spectrum of the nucleus, and pro-
ceeds : —
'When the nucleus was examined in the telescope, it
349
THE WORLD OF COMETS.
appeared as a well-defined minute point of light, of great
brilliancy. I suspected at times a sort of intermittent flashing
in the bright point. The nucleus suggested to me an object
on fire, of which the substance was not uniform in composition,
so that at intervals it burned with a more vivid light. On
July 6 the diameter of the nucleus, when measured with a
power of 800, was l"-8; on July 13 the measure was nearly
double, viz., 3" ; but at this time the point of light was less
defined. On July 15 the nucleus appeared elongated towards
the following side of the comet, at an angle of about 40° to the
comet's axis. The nucleus appeared of an orange colour. This
may be due in part to the effect of contrast with the greenish
light of the coma.'
' The continuous spectrum which accompanies the gaseous
spectrum was observed in every part of the coma ; near its
boundary, and in the dark space behind the nucleus, the con-
tinuous spectrum became so faint as to be detected with
difficulty, at the same time that the bright bands were
distinctly visible. The more distant parts of the tail gave
probably a continuous spectrum only.'
Mr. Huggins thus concludes his remarks: —
' On several evenings I satisfied myself that polarised light
was present in every part of the comet. I do not think that
the proportion of polarised light exceeded one-fifth of the
total light. The polarisation, as exhibited by the partial ex-
tinction of one of the images formed by a double-image prism,
appeared to be more marked in the tail. It must be remem-
bered that such would appear to be the case to some extent
even if the proportion were not really greater, because the
same proportional diminution in a faint object is more appre-
ciated by the eye. Still there was probably a relatively large
proportion of polarised light in the tail.
' The reflected solar light would account for a large part of
350
THE COMET OF 1874, OR COGGIA'S COMET.
the continuous spectrum. To what source are we to ascribe
the remaining light which the prism resolves into a continuous
spectrum? Is it due to reflexion from discrete particles, too
large relatively to the wave-lengths of the light for polarisa-
tion to take place? or is it due to incandescent solid particles?
From the co-existence of the band-spectrum, we can scarcely
think of distinct masses of gas dense enough to give a con-
tinuous spectrum.
4 The difficulty which presents itself in accounting for
sufficient heat to maintain this matter and the nucleus in a
state of incandescence has also to be encountered in respect
of the gaseous matter which emits the light which is resolved
into the bright bands.
'The solar radiation to which the comet was subjected
would be inadequate to account for this state of things directly.
Is there chemical action set up within the comet by the sun's
heat? Is the comet's light due to electricity in any form
excited by the effect of the solar radiation upon the matter of
the comet? Are we to look for the source of the light to the
friction of the particles of the cometary matter which has
been thrown into violent agitation by the comet's approach to
the sun ? ' *
Mr. Christie found that on July 3, 6, 7, arid 13 the tail and
coma were partially polarised in a plane through the axis of
the tail.
On p. 340 M. Guillemin calls attention to the difference
between M. Rayet's drawing of Coggia's comet on July 14 and
Mr. Ne wall's drawing made at the same time, and adds that
the former appears to merit entire confidence. I do not wish
for a moment to say otherwise, but it is only fair to state that
* Mr. Huggins's other papers on cometary spectra are to be found in Proc.
Royal Soc., vol. xvi., p. 386 (1868); vol. xix., p. 488 (1871) ; vol. xx., p. 45
(1872) ; and Phil. Trans., vol. clviii., p. 555 (1868).
THE WORLD OF COMETS.
Mr. Newall's drawing is confirmed by nearly all those of the
same date that I have seen. It will be found engraved in the
E.A.S. Notices for March 1876, and in the letter accom-
panying it Mr. Newall writes : 'The next view we got of the
comet was on Tuesday, July 14, when a wonderful change had
taken place. This is extremely well represented by fig. 2,
which was made by Mrs. Newall, and was so exact that I did
not touch it. Here the nucleus is still very distinct. The two
streams of the tail have separated and become shorter, leaving
a wider dark space between them, while from the two corners
of the fan proceed two antennae, which appear to be projections
of the inner sides of the tails, and preceding these is a luminous
cloud.'
M. Guillemin's description of the drawing is very exact, viz.
the two parabolic arcs start from the nucleus, and being sym-
metrical with regard to the axis of the tail, resemble very
closely the antennae of a moth. In the drawing by Mr. Plummer
of the comet on July 14,* as seen in a refractor of 10 inches
aperture, the two arcs are also placed symmetrically with regard
to the axis; and a sketch made by M. Dreyer at Copenhagen, f
on July 13, with an 11 -inch refractor, also shows the same
arrangement.
The drawing of Mr. With J represents a fan-shaped struc-
ture, with the apex at the nucleus and slightly inclined to the
axis ; and there is but a very slight want of symmetry shown
in the drawings of Mr. Wilson § or of Mr. Huggins. || The
drawing of Dr. Vogel for July 14^f does not show the interior
structure very clearly.
It is well known how greatly the drawings of astronomical
appearances, as seen in different telescopes, may differ from
* R.A.S. Notices, Dec. 1874. § R.A.S. Notices, Dec. 1874.
t Id., May 1876. || Proc. Royal Soc., vol. xxiii., p. 159.
J Id., March 1876. f Ast. NacJt., No. 2,018.
S68
THE COMET OF 1874, OR COGGIA'S COMET.
one another, (as, for example, is the case with the careful
delineations of the great nebula in Orion, by Lord Rosse, Mr.
Lassell, and Father Secchi), and, making some allowance for
this, I have been struck with the generally close agreement in the
representations of Coggia's comet, the most exceptional of which
seems to me to be that of M. Rayet, shown in fig. 60. The
duplicate structure, resembling two parabolas superposed the
one over the other, is mentioned by several observers, and the
chief difference in the drawings is that while M. Rayet inclines
the two parabolas to one another at an angle, most of the
other observers agree with Mr. Newall in making them cross
one another symmetrically. The fact that Mr. Newall alone
saw the plumes reaching right up to the nucleus is, doubtless,
due to the great aperture of his telescope.
Coggia's comet was seen at the Observatory, Melbourne, by
Mr. R. L. J. Ellery, on the morning of July 27. A series of
drawings was obtained by means of the Great Reflector : the
comet was very bright, and the nucleus very stellar. It had
much diminished in brightness by August 10. On October 7
it was still visible, but was too faint for smaller telescopes than
the Melbourne Reflector.
The comet was observed by Mr. Tebbutt at Windsor,
New South Wales, from August 1 till October 7. It was a
very conspicuous object during the first week of August, and
was still faintly visible to the naked eye at the end of that
month. It was also seen on July 27 by Mr. A. A. Anderson,
in the eastern hemisphere (in lat. 23° 30' S., long. 28° 54' E.),
when travelling to Barkly, Griqualand West, South Africa.
It was very bright, and the tail was apparently short, but
this was partially owing to the brightness of the moon. Mr.
Anderson made observations with a sextant from July 27 to
August 8.
It does not appear that any signs of the division or dis-
353 A A
THE WORLD OF COMETS.
ruption of the comet were noticed in the southern hemisphere.
Mr. Ranvard has remarked that when the comet became visible
in the southern hemisphere ' the inner duplicate structure was
still visible, but the outer arcs had been dissipated :' so that
the comet does not seem to have undergone any marked
changes in consequence of its passage near the sun.
364
CHAPTER XI.
THEORY OF COMETAEY PHENOMENA.
355 A A 2
SECTION I.
WHAT IS A COMET ?
Complexity and extent of the question — The law of gravitation suffices to explain the
movements of comets — Lacunae in the theory ; acceleration of the motion of the
comets of Encke and Faye — Origin of comets ; their systems — Questions relative
to their physical and chemical constitution — Form of atmospheres ; birth and
development of tails.
LET us glance back for a moment at the contents of the
preceding chapters.
We there find many facts accumulated, observations both
interesting and instructive, phenomena whose variations suggest
reflections without limit concerning the nature of the bodies to
which they relate. Nevertheless, do these collected facts per-
mit a clear and certain reply to the simple question: What is
a comet?
I say a simple question, for so, as a rule, it is thought to be
by non-scientific people; but in reality there is no question
more complex. In order to attempt to reply to it, or at least
to relate what is known for certain about comets, and to pass
in review the most probable conjectures on doubtful points, we
must proceed methodically, and thus as it were divide the
difficulty.
A first natural division of the subject is at once apparent,
it seems to us, from the exposition of cometary phenomena
which has been made in the preceding chapters. This
357
THE WORLD OF COMETS.
division includes the movements of cornets, either apparent
or real, all that relates to their orbits, and, in a word, the
laws which govern them, not only as concerns what we may
call the regular portion of their course, but in the vicissi-
tudes and perturbations to which they are subjected by other
celestial bodies. So far — in theory, at least — we find no
difficulty in explaining the various facts, such as the periodi-
city of certain comets, the disappearance of some, the non-
reappearance of others, the delay or too speedy arrival of
those whose epoch of return has been calculated. Gravitation
is the principle that renders an account of all these facts, of all
these movements ; the theory of comets is, in this respect,
the same as that of the planets ; and, if there still remain
difficulties and facts unexplained, neither the principle nor
its application are for a moment doubted by any true
astronomer.
There are difficulties, as we have already seen. For
example, we ask ourselves, under the operation of what cause
does Encke's comet continually shorten its period of revolution ?
Is this diminution due to the influence of a resisting medium
or to the action of a repulsive force ? Opinions are divided ;
but this is no impeachment of the principle of gravitation, or
the fact that the sun attracts a mass inversely as the square of
its distance from his centre.
There are obscurities, as, for instance, the origin of comets.
That all comets belong to the solar system cannot be sup-
posed, as certain amongst them move in hyperbolic orbits.
But have all these bodies come originally from beyond
the limits of the solar system? Do they form, as M. Hoek
believes, groups or systems ; and are we to consider the con-
version of their original orbits into closed orbits as due to the
disturbing action of the planetary masses? These questions
are not yet decided ; but, whatever the reply that science may
358
WHAT IS A COMET ?
hold in store, it is certain that they affect in no respect either
the cause of the cometary movements or their laws.
Lastly, there are unexplained facts, such as the non-
reappearance of the comet of 300 years' period (that of
1264-1556), the non-return of some of the comets of short
periods, and the division of Biela's comet into two distinct
comets.
But this last and very curious phenomenon may perhaps
have been due to the action of external forces, and in this
case it would belong to the second or physical category of
problems involved in the question before us.
The mass, the density, the physical constitution of the
luminous nucleus, of the atmosphere surrounding it, and of the
matter which streams out from it as the comet draws near
the sun ; the variations of form and volume of the nucleus and
the nebulosity ; the singular transformations which are revealed
to us by the telescope, more especially those relating to the
origin, development, and disappearance of the tail, are all facts
that have been well and carefully observed, as the preceding
chapters testify, but which are nevertheless difficult to co-
ordinate into a logical whole and to reduce to a single prin-
ciple, from which all the observed facts could be deduced as so
many particular consequences. The phenomena presented to us
by these bodies indicate that they have a special constitution of
their own, as has been justly remarked by M. Roche, the
author of some researches of the highest interest that we shall
shortly proceed to analyse. In the meanwhile we will give
M. Roche's views on the subject before us : —
' Comets are characterised less by the form and position of
their orbits than by the changes they submit to during the
times of their apparitions, and which sometimes succeed each
other with wonderful rapidity. These changes denote a
physical condition peculiar to comets, and mark important
359
THE WORLD OF COMETS.
distinctions between them and other celestial bodies. Whilst
the centre of gravity of the comet is describing its trajectory
around the sun, under the influence of the solar gravitation
and the disturbing action of the planets it may happen to
approach, the comet itself experiences important changes, in
which it is impossible not to recognise the action of the sun;
for it is chiefly in the neighbourhood of the perihelion that
these modifications are developed upon the grandest scale.'
M. Roche divides the phenomena in question into two
kinds — those relating to the tail, its appearance, its varied form,
its brightness and extent; and those which have reference to
O '
the variations of form or luminous intensity of the parts which
constitute the head. The latter, as we have seen, are pheno-
mena which have only been observed in comparatively recent
times, whilst the formation of the tail has been long studied. To
the explanation of these appendages, considered as the principal
characteristic element of comet ary bodies, astronomers have
devoted many efforts. The hypotheses arising from these
attempts are numerous ; but they may be distinguished into
four principal hypotheses, which we will now proceed to de-
scribe successively.
300
SECTION II.
CARDAN'S HYPOTHESIS.
Cometary tails considered as effects of optical refraction— Objections made by Newtr.n
and Gregory — New theory of Gergonne : ideas of Saigey on the subject of planetary
tails — Difficulties and lacunae in this theory.
PANJSTIUS, a philosopher of antiquity, held the belief that
comets did not really exist, but were false appearances. 'They
are,' he says, ' images formed by the reflexion, in the heavenly
expanse, of the rays of the sun.' In the opinion of Cardan
and some astronomers and physicists, Apian, Tycho Brahe*,
in the Renaissance, and Gergonne and Saigey, in our time, the
tails of comets are simple optical appearances.
The following is the passage in Cardan's work (De Sub-
tilitate) which relates to this question : ' It is, therefore, evident
that a comet is a globe situated in the heavens and rendered
visible by the illumination of the sun ; the rays which pass
through it form the appearance of a beard or tail.'"5'5 The
Milanese doctor has entered into no particulars respecting the
manner in which these appearances are formed, which, in his
opinion, were doubtless analogous to the effects of refraction
produced by the convergence of luminous rays passing
* ' Quo fit ut clare pateat cometem globum esse in caelo constitutum, qui a
sole illuminatus videtur, et dum radii transeunt; barbse aut caudse effigiem
formant.' (De Subtilitate, lib. iv. 118; edition 1554.)
361
THE WORLD OF COMETS.
through a lenticular glass or globe filled with water; .such
as were formerly employed by artisans for concentrating the
light upon their work.
* But, as Newton and Gregory have remarked,' justly
objects M. Roche, ' the light is only visible in proportion as it
reaches the eye ; it would be necessary, therefore, that the
solar rays, refracted by the head of the cornet and collected
behind in one convergent beam, should be sent towards the
earth by material particles. Thus, sometimes when the sun is
near the horizon and hidden by clouds, its rays, reflected by
particles of air or vapour, are seen clearly denned against the
sky like luminous jets.'
The fundamental idea upon which this explanation rests,
and which still bears the name of Cardan, has since been
several times taken up and modified. Following in the same
order of ideas, we shall mention only a memoir by Gergonne,
entitled Essai analytique sur la nature des queues des cometes.
The origin of the phenomenon is there considered as purely
optical, tails being only an appearance due to the most
illuminated portion of the cometary atmosphere, or, more
correctly speaking, to the caustic surface which is the envelope
©f the solar rays that are refracted whilst traversing the
nucleus or its surrounding layers. These rays become visible
when reflected upon the particles which compose the atmo-
sphere of the comet. But, according to this theory, the
atmosphere should have a radius at least equal to the length
of the tail ; and this constitutes an almost insurmountable ob-
jection, which the author himself does not conceal. We must
not forget that certain comets have had tails many millions of
miles in length. The atmospheres of comets being certainly
much more limited, the reflexion, we must suppose, takes
place upon the particles of an interplanetary medium,
independent of the comet itself, and extending to distances
362
CARDAN'S HYPOTHESIS.
far beyond the limits of the zodiacal light. Saigey, in his
Physique du Globe, admits this explanation of cometary tails,
and, according to him, planets have virtual tails, which
would become real 'if the interplanetary spaces were filled
with a matter similar to that which accompanies these
last-mentioned bodies.' Saigey, it is evident by this last line,
believes in the indefinite extension of cometary atmospheres,
and the objection made above exists in full force. As regards
the form of tails, their curvature, their multiplicity, and their
oscillations, he explains them as follows : the curvature, by an
effect of aberration due to the finite velocity of the propa-
gation of light, the multiplicity by irregularities in the form
of the nucleus, the oscillations by a movement of rotation,
which causes these irregularities to be periodic.*
According to this system, new almost entirely abandoned, it
is difficult to explain the phenomena of which we have already
given an account, and which are seen by the telescope to vary
from hour to hour; we mean the development of luminous jets
in front of the nucleus, together with the envelopes and the
lateral reflux of the luminous matter to form the edges of the
tail. Nor is it less difficult to explain the formation of tails,
which have sometimes been projected towards the sun, unless,
not content with comparing comets to transparent and refract-
ing globes, we are willing to make them perform at the same
time the part of concave mirrors.
We have seen that the observations of occupations of stars
by cometary atmospheres do not furnish proof of any refracting
power whatever in the nebulosity of the head. It must,
* Speaking of the tail of the earth, he observes : ' The axis of the luminous
sheaf ought to have, mathematically, the form of a spiral of Archimedes, the
generating circle of which is 64,000 times greater than the terrestrial orbit; so
that the most brilliant portion of this sheaf is slightly curved in the rear of the
earth's movement of translation.'
363
THE WORLD OF COMETS.
therefore, be the refraction due to the nucleus alone that gives
rise to the formation of caustics, and thus produces, at a dis-
tance, the illusion of cometary tails. We have said enough,
however, of an hypothesis which has little chance of rising
again from the discredit into which it has fallen.
364
SECTION III.
THEORY OF THE IMPULSION OF THE SOLAR RAYS.
Ideas of Kepler concerning the formation of tails — Galileo, Hooke, and Euler — Hyp~>-
thesis of Kepler formulated by Laplace — Where does the impulsion come from in
the theory of undulations ?
KEPLER, who for a moment suffered himself to be led away
by the idea of Cardan,* soon abandoned it, and substituted in
its place that of the action of the solar rays. According to
this theory cometary tails have substance and are formed of
materials borrowed from the comet, its nucleus, or at least its
nebulosity. ' The sun,' says Kepler, ' strikes upon the spherical
mass of the comet with direct rays, which penetrate its substance,
and carrying with them a portion of this matter, issue thence
to form that trace of light which we call the tail of the comet.
* It appears that Galileo was also a partisan of the same theory. ' We find,'
says Arago, in a work entitled II Trittinatore, ' that Galileo gave it his appro^
bation.'
[It is worthy of remark that Kepler seems to have abandoned Cardan's
theory mainly because it failed to explain the curvature of tails. He remarks
that the laws of optics teach us that the paths of light-rays are rectilinear, so
that, if produced as supposed by Cardan, the tail could not be curved. But if
we take into account the fact that the velocity of light is finite — a fact not
known in Kepler's time — it is easily seen that the tail will appear curved except
when the earth should happen to be in the plane of the comet's orbit. M.
W. de Fonvielle has recently called attention to this point in the history of
Cardan's theory. See Monthly Notices of the Royal Astronomh.il Society, vol.
xxxv. p. 408. 1875.— ED.] _
365
THE WORLD OF COMETS.
This action of the solar rays rarefies the particles which com-
pose the body of the comet ; it drives them away and dissi-
pates them.'
Hooke, a contemporary of Newton, in order to explain the
ascent of the light and tenuous matters which, emanating from
the nucleus and flowing back in a direction opposite the sun,
contribute to form the tail, assumes that these volatile matters
are imponderable : to gravitation he opposes their levitation ;
according to him they have a tendency to fly from the sun.
This amounts to assuming a repulsive force without explaining
where this force resides.
The opinion of Kepler has been completed, extended, and
modified. Admitted by Euler, and then by Laplace, it may be
considered as the starting-point of the theory maintained by
several contemporary astronomers, and notably by M. Faye —
the theory according to which the solar rays exercise a re-
pulsive action at a distance. We shall devote to it in its
present form a separate section. In the meantime let us see
how this theory has been formulated by Laplace in his Ex-
position du Systeme du Monde : —
* The tails of comets appear to be composed of the most
volatile molecules which the heat of the sun raises from their
surface and by the impulsion of his rays banishes to an
indefinite distance. This results from the direction of these
trains of vapour, which, always situated, as regards the sun,
on the further side of the head of the comet, increase in pro-
portion as the comet draws near to the sun, and only attain
their maxima after the perihelion passage. The extreme
tenuity of the molecules increasing the ratio of the surface to
the mass, the impulsion of the solar rays becomes sensible, and
causes nearly every molecule to describe a hyperbolic orbit,
the sun being the focus of the corresponding conjugate
hyperbola. The series of molecules moving in these curves
306
THEORY OF THE IMPULSION OF THE SOLAR RAYS.
form, beginning from the head of the comet, a luminous train
in a direction opposite the sun, and slightly curved towards
that region which the comet has just quitted, whilst advancing
in its orbit ; and this is what observation shows to us to be
the case. The rapidity with which cometary tails increase
enables us to judge of the extraordinary velocity with which
these molecules ascend. The different volatility, size, and
density of the molecules must needs produce considerable
differences in the curves which they describe: hence arise the
great varieties of form, length, and breadth observed in the
tails of comets. If we suppose these effects combined with
others which may result from a movement of rotation in
the comet itself, and the apparent changes arising from the
illusions of the annual parallax, we may partly conceive the
reason of the singular phenomena presented by the nebulo-
sities and tails of comets.'
This hypothesis, it is evident, supposes two modes of action
of the emanations from the sun. The first, which Kepler
vaguely indicated, is an effect of dilatation due to the calorific
activity of the solar rays, an effect doubtless itself preceded by
an evaporation throughout the liquid parts of the surface of
the nucleus. The nebulosity thus becomes more volumi-
nous and the layers which form it more and more attenu-
ated. Up to this point there is no difficulty — the known
physical effects of heat justify this portion of the theory.
The difficulty begins when it is necessary to assume that the
same rays which have hitherto acted as calorific rays are
endowed with another property, hitherto unknown, that
of giving an onward motion or propulsion to all molecules
reduced to a state of suitable tenuity. Does such a force
exist ?
To Laplace, who was justified in adopting the theory of
emission at the epoch when he wrote, this repulsive force was
367
THE WORLD OF COMETS.
quite natural. The luminous molecules emitted by the sun,
moving with enormous speed, communicated a portion of
their momentum to the molecules emitted by the comet, to
those which by the action of heat had been previously reduced
to a sufficiently small tenuity, and hence the formation of the
tail. But it is less easy to conceive of this repulsive force in
the wave-theory of light, now universally adopted. Undula-
tions are propagated with enormous velocity in the ether,
but the matter is not transported forward.* It is difficult
to see how the force that gives rise to the successive waves
can produce the rectilinear movement of the molecules of
the cometary atmosphere. Moreover, before assuming the ex-
istence of an actual repulsive force, it should, if possible, be
demonstrated by experiment. Arago, when citing the ex-
periments of Homberg, opposes to them the negative veri-
fications of Bennet, and concludes thus : ' The fundamental
idea of an impulsion due to the solar rays is, therefore, only
an hypothesis, without real value.' The question, however,
is not yet settled, as we shall see.
* M. Roche cites the following comparison, due to Euler, and by which
that great mathematician, a partisan of the theory of luminous waves, justifies
his adhesion to the hypothesis of the impulsion of the solar rays : —
'As a violent sound excites not only a vibratory movement in the particles
of air, but also causes a real and perceptible movement in the light dust floating
in the atmosphere, we cannot doubt that in the same way the vibratory motion
caused by light produces a similar effect.'
This very vague comparison is not conclusive. Sonorous waves have an
amplitude sufficient to produce visible agitation ; the fact that we. have to prove
is the existence of a progressive rectilinear movement, not the existence of
oscillation.
368
SECTION IV.
HYPOTHESIS OF AN APPARENT REPULSION.
Views of Newton on the formation of the tails of comets — Action of heat and rarefac-
tion of the cometary matter — The ethereal medium, losing its specific weight, rises
opposite the sun, and carries with it the matter of the tail — Objections which have
been made to the hypothesis of a resisting and ponderable medium.
NEWTON, in order to explain the formation of the tails of
comets, had recourse to no other causes than the ordinary action
of the calorific rays on the one hand, and that of gravitation on
the other. But, although he does not introduce any new force,
he is obliged to suppose that the comet during the whole time
that its tail is developing is traversing a medium subject to the
force of gravitation and tending towards the sun. Newton
thus explains the theory : —
The tail is composed of vapours, that is to say, of the
lightest parts of the atmosphere of a comet. These vapours
are rarefied by the action of the solar heat, and in their turn
heat the surrounding ether. Thus, the medium which sur-
rounds the comet becomes rarefied ; it consequently loses its
specific weight, and instead of tending with the same energy
towards the sun, it continues to rise in the same manner as
layers of air heated at the surface of the soil rise in virtue
of the principle of Archimedes. In rising it carries with it
particles of cometary matter, which by their ascension produce
the tail, rendered visible by the reflexion of light proceeding
369 B B
THE WORLD OF COMETS. %
from the sun. In this manner smoke ascends in a chimney by
the impulsion of the air in which it is suspended ; this air is
rarefied by the heat ; it ascends because its gravity or spe-
cific weight has become less, and it draws along in its ascent
columns of smoke. The ascension of cometary vapours further
arises from the fact that they revolve about the sun, and for
this reason have a tendency to fly from it. The atmosphere
of the sun or matter of the heavens is at rest or turns slowly,
having received its movement of rotation from the sun. Such
are the causes which determine the ascent of cometary tails in
the vicinity of the sun where the orbits are much curved, and
where the comet, plunged in a dense and consequently heavier
atmosphere, emits a longer tail.
This theory, which had been in the first place vaguely
formulated by Riccioli, and then by Hooke (the latter, we may
remember, inclines rather to the doctrine of a repulsive force),
was adopted by different astronomers of the eighteenth century,
Boscovich, Gregory, Pingre, Delambre, Lalande. Gregory, how-
ever, was not contented with the cause assigned by Newton for
the ascent of cometary tails ; he believed also in an active im-
pulsion in addition to an apparent repulsive force. His system
is a combination of the two systems we have just described.
Various objections of a serious kind have been urged
against the theory of Newton. The existence of a resisting
medium, gravitating towards the sun, of a solar atmosphere,
in fact, would necessarily be limited to within a certain dis-
tance of the sun himself. Laplace has proved that for such
an atmosphere to subsist it must be animated by a movement
of rotation about the sun's axis, and that it could not extend
beyond the distance at which the centrifugal force arising from
that movement would become equal to the force of gravitation. In
the plane of the solar equator the limit is seventeen hundredths
of the mean distance of the earth; it corresponds to the radius
370
HYPOTHESIS OF AN APPARENT REPULSION.
of the orbit of a planet whose revolution would be equal in
duration to the solar rotation, which is effected in twenty-five
days and a half. Now, comets, before attaining such proximity
to the sun, are provided with tails ; and considerable tails have
been exhibited by comets whose perihelion distance has even
exceeded the radius of the terrestrial orbit, which is nearly six
times as great as the extreme possible limit of the solar atmos-
phere.
Besides, this ponderable medium would be a resisting
medium as well. In addition to the disturbing action that this
resistance would exercise upon the head of the comet and
likewise upon its orbit, it would act with much more intensity
upon the tail of the comet, on account of its extreme rarity.
Before the perihelion passage, in the first part of the comet's
movement, the curvature and the drifting back of the tail
would be easily explained by this resistance ; but, after the
perihelion passage, the tail continues to keep the same position
relatively to the radius vector joining the nucleus to the sun, so
that the comet appears to move its tail to a position in advance
of itself, a phenomenon incompatible with the hypothesis of a
resisting medium. The medium of which we speak has like-
wise been assimilated to the zodiacal light; and Mairan, who
has thus explained the terrestrial Aurora Borealis, finds in
this light the cause and origin of cometary tails. But the
preceding objections and others, which would take too long to
repeat here, have been justly opposed to this new theory.
371 B B 2
SECTION V.
THEORY OF OLBERS AND BESSEL.
Hypothesis of an electric or magnetic action in the formation of tails — Repulsive
action of the sun upon the cometary matter, and of the nucleus upon the nebulosity
—Views of Sir John Herschel and M. Liais — Theory of Bessel— Oscillations of
luminous sectors — Magnetic polar force.
WHETHER the cause which determines the production of co-
metary tails and their development, at once so immense and
so rapid, be a force sui generis, or only an apparent force, it is
none the less true that it has all the features of a repulsive
action or force. Heat, the impulsion of the solar rays, gravi-
tation, have all been variously combined in order to furnish the
desired explanation; it evidently remained to try the interven-
tion of the electric and magnetic forces.
From this point of view Olbers, Herschel, and Bessel have
in turn applied themselves to the problem. We will give a
brief analysis of the opinions held by these illustrious
astronomers.
The comet of 1811 first drew the attention of Olbers to the
subject. ' This astronomer,' says M. Roche, ' attributes to the
proximity of the comet and the sun a development of electricity
in both these bodies ; hence arises a repulsive action of the sun
and another repulsive action of the comet upon the nebulosity
which surrounds it.' By the first of these forces Olbers has
372
THEORY OF OLBERS AND BES8EL.
explained the formation and development of tails ; by the
second he has accounted for the formation of the luminous
sectors or plumes of the comet, and also the successive
envelopes similar to those which were observed in Donati's
comet. Biot has given his adhesion to this theory.
Sir John Herschel's view is nearly the same. It is not
improbable, he observes, that the sun is constantly charged
with positive electricity ; that as the comet draws near the sun
and its substance becomes vaporised the separation of the two
electricities takes place, the nucleus becoming negative and the
tail positive. The electricity of the sun would direct the
movement of the tail, just as an electrified body acts upon a
non-conducting body electrified by influence.*
* In his Outlines of Astronomy Sir J. Herschel is less explicit in regard to
the physical nature of the force which produces the tails, and he does not refer
to electricity. But, speaking of the curious phenomena which were observed to
take place in the head of Halley's comet during its apparition of 1835, he
proceeds : —
' Reflecting on these phenomena, and carefully considering the evidence
afforded by the numerous and elaborately executed drawings which have been
placed on record by observers, it seems impossible to avoid the following con-
clusions : —
' 1st. That the matter of the nucleus of a comet is powerfully excited and
dilated into a vaporous state by the action of the sun's rays, escaping in streams
and jets at those points of its surface which oppose the least resistance, and in all
probability throwing that surface or the nucleus itself into irregular motions
by its reaction in the act of so escaping, and thus altering its direction.
' 2ndly. That this process chiefly takes place in that portion of the nucleus
which is turned towards the sun, the vapour escaping freely in that direction.
' 3rdly. That when so emitted it is prevented from proceeding in the direction
originally impressed upon it by some force directed from the sun, drifting it
back and carrying it out to vast distances behind the nucleus,, forming the tail,
or so much of the tail as can be considered as consisting of material substance.
' 4thly. That this force, whatever its nature, acts unequally on the materials
of the comet, the greater portion remaining unvaporised ; and a considerable
part of the vapour actually produced remaining in its neighbourhood, forming
the head and coma.
' 5thly. That the force thus acting upon the materials of the tail cannot pos-
sibly be identical with the ordinary gravitation of matter, being centrifugal or
373
THE WORLD OF COMETS.
M. Liais, in his work entitled LEspace Celeste, has pro-
nounced in favour of a repulsive force of an electric nature.
According to this astronomer the calorific action of the solar
rays causes a physical and chemical modification of the
molecular condition of the nucleus, and thus gives rise to the
two electricities. Whilst the nucleus is charged with the one
electricity, the opposite electricity becomes developed to the
more attenuated and lighter portions, and is carried by them to
the limits of the cornetary atmosphere. But, on the other hand,
the sun himself is constantly in a state of strong electric tension*
And of the two electricities that which he possesses in excess
will attract, for example, the nucleus, if it should be charged
repulsive as respects the sun, and of an energy very far exceeding the gravi-
tating force towards that luminary. This will be evident if we consider the
enormous velocity with which the matter of the tail is carried backwards, in
opposition both to the motion which it had as part of the nucleus and to that
which it acquired in the act of its emission, both which motions have to be
destroyed in the first instance before any movement in the contrary direction
can be impressed.
' Gthly. That unless the matter of the tail thus repelled from the sun be
retained by a peculiar and highly energetic attraction to the nucleus, differing
from and exceptional to the ordinary power of gravitation, it must leave the
nucleus altogether ; being in effect carried far beyond the coercive power of so
feeble a gravitating force as would correspond to the minute mass of the nucleus;
and it is, therefore, very conceivable that a comet may lose, at every approach
to the sun, a portion of that peculiar matter, whatever it may be, on which the
production of its tail depends, the remainder being of course less excitable by
'the solar action, and more impassive to his rays, and therefore, pro tanto, more
nearly approximating to the nature of the planetary bodies.
' 7thly. That, considering the immense distances to which at least some
portion of the matter of the tail is carried from the comet, and the way in which
it is dispersed through the system, it is quite inconceivable that the whole of
that matter should be reabsorbed ; that, therefore, it must lose during its peri-
helion passage some portion of its matter ; and if, as would seem far from impro-
bable, that matter should be of a nature to be repelled from, not attracted by,
the sun, the remainder will, by consequence, be, pro quantitate inertia, more
energetically attracted to the sun than the mean of both. If, then, the orbit be
elliptic, it will perform each successive revolution in a shorter time than the
preceding, until, at length, the whole of the repulsive matter is got rid of.'
374
THEORY OF OLBERS AND BESSEL.
with the opposite electricity, and will, in the same way, repel
the molecules of the electrised atmosphere.
This expulsion, exercised over a portion of the atmosphere
which already has a tendency to fly from the sun, originates a
tail nearly opposite to the radius vector ; but, acting upon the
anterior molecules, it drives them back and causes the for-
mation of a second tail. ' As the electric action of the sun,' he
observes, 'exerted as an attractive force upon the nucleus, and
as a repulsive force upon the nebulosity, somewhat disturbs the
figure which the comet would assume under the sole influence
of gravitation, and causes a much greater extension of the ne-
bulosity in the rear of the nucleus than on the side near to the
sun, the gravitation of the molecules towards the nucleus is
less on the side opposite to the sun than on the other, added
to which the electricity of a similar kind to that of the comet
is there more abundant, and there consequently the repulsive
force is manifested with the greatest energy. Besides, the
currents of matter are directly in the direction of this force,
and have not to curve themselves back like those that arise
in front. The posterior tail is, therefore, more energetically
repelled than the tail produced in the anterior region. For
this reason it makes with the prolongation of the radius
vector of the comet a less angle, which accords with what has
been found by observation. For the same reason also the
tail which is opposite to the sun is generally the first to ap-
pear and the last to disappear. M. Liais assumes that this
double tail, which was very observable in the comet of 1861,
exists in nearly all comets, and that the two parts of which
the tail is composed appear distinct only when they are very
long, unless, from the position of the earth in their common
plane— the plane of the orbit — they should be projected one
upon the other.'
The preceding will suffice to give a general idea of the
375
THE WORLD OF COMETS.
above theory, which differs in no essential particular from
that of Olbers and Sir John Herschel. M. Liais has entered
into details top long to be here reproduced, by means of which
he proceeds to explain all observed anomalies, and to show,
with perhaps a little too much confidence, that these appa-
rent anomalies are legitimate consequences of the theory of
cometary electricity. Tails multiple in number, straight or
curved, plume-shaped or fan-shaped, luminous sectors and
jets, tails directed towards the sun, the double curve of the
tail of the comet of 1769, the accelerated movement of certain
comets, the rapid and transient coruscations of tails, all corre-
spond exactly, according to M. Liais, with this hypothesis.
In a memoir on the physical constitution of Halley's
comet the illustrious astronomer Bessel has formulated a
theory not very different to that of electricity. The end
which he chiefly had in view was the explanation of the
luminous aigrettes observed in 1835,^a phenomenon which has
since been repeated and observed in the head of the great cornet
of 1862. Bessel compares the axis of the comet to a magnet,
one of whose extremities or poles is attracted to the sun,
whilst the other extremity is repelled. To an equilibrium by
turns broken and established under the action of internal
forces, and the polar force proceeding from the sun, the
observed oscillation is due. This polar force, in the part of its
action which is repulsive, tends to form and develop the tail.
' As regards the physical origin of the force,' says M. Roche,
in his analysis of Bessel's memoir, ' he attributes it to a
particular action of the sun which accompanies the volatili-
sation of the cometary fluid, and tends at the same time to
repel each molecule of matter and to direct the axis of the
comet towards the sun. This force would not be proportional
to the mass, but specific ; that is to say, it would act with dif-
ferent intensity upon different matters. This specific character
376
THEORY OF OLBERS AND BESSEL.
would explain the production of multiple tails. The action
of the polar forces as conceived by Bessel is very compli-
cated ; their influence upon the nucleus of the comet is obscure,
and renders impossible any kind of equilibrium in the atmos-
phere by which it is surrounded.'*
* The following is a resume of Bessel's memoir, taken from the notice of it
inserted in 1840 in the Connaissance des Temps : —
' The illustrious astronomer, in the first place, describes the appearances
presented by the head of the comet between the 2nd and the 25th of October,
1835, dwelling more especially upon the movements of the aigrette, of " that
effusion of luminous matter which issued from the nucleus and was directed
towards the sun. The most curious phenomenon presented by the comet," he
observes, u was unquestionably the movement of rotation or oscillation of the
luminous cone." He then endeavours to determine in what manner and in what
direction this movement was effected, and from observations and measures of the
position of the aigrette he was led to consider probable the hypothesis of a pen-
dulous motion of the luminous cone in the plane of the orbit of the comet and
about an axis perpendicular to this plane. The duration of the period was
found to be comprised between four and five days, and the amplitude of the oscil-
lations had a mean value of 60 degrees.
1 Now, how are we to explain this movement ? Could not the attraction of
the sun, acting unequally upon the particles of the comet, more or less distant from
the sun, and combining with the movement of the comet in its orbit, produce
in the nucleus a libration analogous to the libration of the moon ? ' To the
question thus raised Bessel has given a reply in the negative, because in that
case the oscillations arising from the attraction of the sun would have been
of very long duration, whilst the period of the oscillations was found, on the
contrary, to have been very short. This leads us to consider the possibility
of a special physical force. He thus explains his ideas on the subject : —
' We must assume a polar force tending to direct one of the rays of the comet
towards the sun, whilst at the same time it directs the opposite ray in a contrary
direction : there is no a priori reason for rejecting the existence of such a force.
The magnetism of the earth supplies us with an example of an analogous force,
although it is not yet proved that it has any relation to the sun ; if this were
tlie case we should see the effect manifested in the precession of the equinoxes.
This force once admitted, the oscillatory movement of the aigrette is easily
explained ; the duration of the oscillations depends upon the magnitude of this
force, and the amplitude upon the initial motion of the molecules. I remark,
moreover, that if the sun exerts upon one portion of the comet a force other than
attraction, this force must be a polar force ; that is to say, a force producing
a contrary action upon another portion of the mass. For if this were not the
377
THE WORLD OF COMETS.
case, the sum of all the actions which the sun exerts upon the mass of the comet
would not be proportional to the mass, and consequently the movements of the
comet according to the laws of Kepler would not correspond to the mass of the
sun as determined by the movements of the planet. Now, observation has
shown no deviation that could be attributed to this cause ; if, therefore, we
can prove that the sun does not act equally upon the whole mass of the comet,
it will be a new argument in favour of a polar force.'
Bessel next investigates analytically the path followed by a particle which,
emitted by the nucleus and escaping from the luminous aigrette in the direction
of the sun, proceeds under the influence of the solar repulsion to turn back
upon its path and to separate from the head of the comet in a direction
opposite the sun ; then, comparing the results of analysis with those of obser-
vation, he accounts for the appearances presented by the tails of different
comets, their curvature, their more or less size. &c.
But all comets are far from presenting thi; same phenomena : for example,
whilst the aigrettes of the comets of 1835 and 1744 issued from a particular
region of the surface of the nucleus, in that of 1811 the luminous matter was
emitted in all directions; in that of 1769 there were two distinct aigrettes,
and the comet of 1807 had two tails. We have seen other examples of these
differences. Bessel explains the first by a simple difference in the value of a
constant ; the others by assuming that the repulsive force of the sun is exerted
with variable intensity, being dependent upon the nature of the different por-
tions of the luminous matter.
We will terminate this analysis by quoting the passage in which Bessel
explains the movement of oscillation which, by principally fixing his attention,
gave rise to his theory : —
4 1 regard,' he says, ' the oscillatory movement of the luminous aigrette of
Halley's comet as an effect of the same force which projects in opposite direc
tions the particles emitted from the nucleus parallel to the radius vector. This
is how I conceive that the force in question acts.'
' The total action of one body upon another may be divided into two parts,
the one part acting equally upon all the particles of the other ; and the second
part of the action composed of different actions exerted over different parts.
When two bodies are far separated from each other, and the action is small, it is
only the first part which becomes sensible, in proportion as the distance
diminishes ; the second part can only have an appreciable value later. Thus,
when a comet approaches the sun after having been very far removed, we per-
ceive the general action which the sun exercises over all its parts. I suppose
that this action consists of a volatilisation of particles, which, moreover, are so
polarised as to be repelled by the sun. The second part of the sun's action may
have for its effect the polarisation of the comet itself and a particular emission
of luminous matter in the direction of the sun. The part of the surface from
which issues the luminous aigrette has a polarisation such that it has a ten-
dency to be attracted towards the sun ; and consequently the particles which
378
THEORY OF OLBEHS AND BESSEL.
compose it having the same polarisation, tend also to approach the sun. But
these particles are moving in a space filled with matter having an opposite
polarisation, and which is constantly being replaced. Thus the two polarisa-
tions neutralise each other, and the particles which compose the aigrette will
acquire the opposite property to that which they had previously, in proportion
as they recede from the comet.'
SECTION VI.
THEORY OF COMETARY PHENOMENA.
Researches ofM. E. Roche upon the form and equilibrium of the atmospheres of celes-
tial bodies under the combined influence of gravitation, solar heat, and a repulsive
force — Figure of equilibrium of a solid mass submitted to gravitation and the heat
of the sun — Comets should have two opposite tails — Completion of the theory
of cometary tides by the admission of a repulsive force, real or apparent —Accord-
ance of the theory so completed with observation.
M. EDOUARD ROCHE has devoted a series of highly interest-
ing memoirs to the discussion of the figure assumed by the
atmosphere of celestial bodies under the action of the forces
of the solar system. He has more particularly given his
attention to the study of cometary atmospheres, and to all
the phenomena which take place in and around cometary masses.
M. Roche begins by reducing the question to its simplest
form. He assimilates a comet to ' an entirely fluid mass,
sensibly homogeneous, and having no movement of rotation.'
The forces which act upon it are the mutual attraction of its
own particles and gravitation towards the sun. For such a
mass to be in equilibrium under the action of these forces it
must have the figure of a prolate spheroid with its centre at the
centre of gravity, and its axis of revolution coincident with
the radius vector from the sun.
Introducing then the motion of the comet towards the sun,
M. Roche examines the modifications that would be occasioned
380
THEORY OF COMETARY PHENOMENA.
in the figure of the atmosphere by a diminution of the dis-
tance between itself and the sun, only taking into account
the mutual attraction of the two bodies. * At first spherical
when the comet is far off, its figure becomes ellipsoidal, and
gradually lengthens as it draws near the sun/ But there is a
limit to the amount of lengthening, a limit which depends
upon the density of the fluid of which the cometary atmos-
phere is formed.
Although, according to M. Roche, there may not exist
amongst the numerous comets of the solar system any one
whose physical constitution accords exactly with the above
hypothesis, nevertheless it is more natural to consider the
general question of a central nucleus surrounded by an atmos-
phere of much greater rarity. In all the atmospheres of the
celestial bodies the gaseous envelope is retained by gravitation
towards the nucleus. Let us see what theory gives on this
new hypothesis : —
' On following with care,' says M. Roche, 'the phenomena
developed by a comet during its approach to the sun, we
clearly see that they result, at least in part, from the increasing
action of the solar gravity. The difference of the attractions
exerted by the sun upon the nearest and the furthest portions
of the cometary atmosphere must needs have the effect of
lengthening the comet in the direction of the sun, and more
and more in proportion as the distance of the comet from
the latter continues to decrease. In a word, the cause that
produces the terrestrial tides will here manifest itself in a
similar manner, but on a much grander scale, in the neighbour-
hood of the perihelion. This has been confirmed by my
mathematical investigations.' In fact, the surfaces of equili-
brium,* originally spherical, lengthen more and more towards
* [A surface of equilibrium, or, as it is often called, a level surface, is a surface
,6uch that if a particle were placed at any point on it, the resultant of the action
381
THE WORLD OF COMETS.
the sun, as by the diminution of the distance From the sun its
action increases ; the comet also lengthens not only towards
the sun, but also to an equal extent in the opposite direction.
But the atmosphere cannot extend further than to the points
where the attractions of the sun and the nucleus exactly
balance one another. Any particle that escapes beyond this
limiting surface of equilibrium, which is called the free surface,
is then subject to the preponderating influence of the sun, and as
it were abandons the cornet. The exterior surfaces of equili-
brium have no longer a spheroidal form: they open themselves
as it were at the two poles, A and A', and consist of sheets
that extend to infinity. (Fig. 63.)
Fig. 63.— M. Roche's theory of cometary phenomena. Limiting atmospheric surface of
equilibrium.
' If, from any cause, the cometary fluid should pass be-
yond the free surface, it will spread itself in all directions
over the surfaces of equilibrium that are immediately exterior;
and, as they are infinite in extent, the fluid in excess will
stream off through the two conical summits or poles as
through two openings and lose itself in space.' (Fig. 64.)
So far gravitation is the only force whose action we have
of the external forces upon the particle would act in a direction perpendicular
to the surface at the point. Thus, at any point within the free, or bounding,
surface of the atmosphere the resultant of the external forces acting upon the
particle of the atmosphere that is situated there is perpendicular to the surface
of equilibrium passing through it, and this resultant is balanced by the pressure
of the atmospheric layers underneath. — ED.]
382
THEORY OF COMETARY PHENOMENA.
taken into account, and we have deduced from it the figure of
equilibrium of a comet, regarded as a homogeneous fluid mass,
or as a nucleus, either liquid or solid, surrounded hy a ponder-
able atmosphere. We may go further still without the in-
tervention of another force; for M. Roche finds that, as
regards the free surface, its dimensions vary with the distance
of the sun : ' this surface, as it were, contracts as the comet
approaches perihelion, and the fluid layer that is thus left
outside it flows away at the two poles, thus forming two
opposite jets along the radius vector of the sun. If the fluid
is elastic and behaves like a gas, the outflow will continue so
long as molecules continue to come from the interior to re-
ig. 64. — Flow of comotary matter beyond the free surface of the atmosphere. No
repulsive force.
place those which escape. This is what must happen whilst
the comet is drawing near the sun.' When the perhelion has
passed this state of things no longer continues.
But another cause will have been contributing, and doubt-
less in a much more powerful degree, to produce this outflow
of cometary fluids from the two extremities of the axis of the
comets, as represented in fig. 64. This cause is the calorific
action of the solar rays upon the nucleus, an action which
continues rapidly to increase in the neighbourhood of the
perihelion. The accumulated heat then gradually dilates and
volatilises the cometary substance which rises by the dimi-
nution of its specific weight, attains the free surface, and,
passing beyond it, flows off into space, as we have just said.
383
THE WORLD OF COMETS.
The whole of this matter which thus abandons, not only the
nucleus, but the cometary atmosphere itself, becomes resolved
into particles independent of the comet, ' into a cloud of dust
composed of an infinite number of disconnected molecules;*
if they fail to disperse and continue still agglomerated, it is
because their motion is nearly the same, owing to their common
initial velocity.'
Such, in its principal features, is the theory which M.
Roche has named the theory of cometary tides. It involves
only the action of gravitation and heat. We must next en-
quire if the phenomena actually observed correspond with the
results of the analysis. As regards the appendages of the
head and the growth and development of tails, does it suf-
fice to furnish an explanation of these phenomena ? To this
question M. Roche replies in the negative. ' If this theory,'
he remarks, ' were sufficient to explain all ; that is to say, if
the attraction of the sun and that of the nucleus were the sole
influences concerned in the production of these phenomena
* This is the expression employed by M. Roche in his memoirs, and it is no
doubt true, if we consider this dust, molecule by molecule ; but is it right to
say that these particles have no longer any connexion with each other ? They
are always under the influence of gravitation, both towards the nucleus and the
sun ; the latter being preponderant, the system of expelled molecules continues
to exist, and for two reasons : firstly, because there is no reason to believe that the
molecules have lost all mutual action ; secondly, because they all continue to gravi-
tate towards the sun, as is shown besides by the apparent solidarity of comets and
their tails. Now, are we to suppose that the whole of this abandoned matter
returns no more to the comet ? It is both possible and probable, if it continues
to be endowed with the property of gravitation (and how could this property
have been lost?), that it may be attracted by the sun or some other planet
encountered on its way, which benefits by the losses of the comet ; but, on the
other hand, as the velocity acquired causes it to follow to a great extent the
movement of the comet in its orbit, it will happen that at a certain distance from
the perihelion the action of the sun ceases to preponderate, and gives place
to that of the nucleus ; the disengaged matter will then regain the limits of
the cometary atmosphere, since the free surface dilates in the second half
of the orbit in proportion as the distance from the sun augments.
384
THEORY OF COMETARY PHENOMENA.
(there is heat as well), a constant agreement would exist
between the phenomena observed and the theoretical results I
have pointed out. Every comet would then have not one tail but
two tails, directly opposite to one another. This circumstance,
which was observable in the comet of 1824 and some others,
is nevertheless exceptional ; as a rule, only th'e tail opposite
the sun is observed. The tail in front is more frequently re-
placed by a brilliant aigrette, which may be considered as a
rudimentary tail. In the aspect of comets there is neverthe-
less so great a want of symmetry, that we are obliged to
acknowledge the insufficiency of our theory of cometary tides ;
it only explains a part of the phenomena.'
He then asks himself if this want of symmetry may not be
explained in part by the unequal action of the solar heat upon
the anterior and posterior portions of the comet. To a certain
extent, yes. But it still remains to explain the extraordinary
length of the tail opposite the sun, the cessation in the de-
velopment of the second tail, the figure of the aigrettes, which
appear to be bent and driven back to contribute to the for-
mation of the tail opposite. A necessity arises, therefore, for
the intervention of a new force, either apparent or real,
which shall exercise a repulsive action upon the matter of the
comet.
When M. Roche published the memoirs above referred to,
Donati's comet had recently attracted the attention of the
astronomical world by the singular phenomena revealed to
observation in the structure of its different parts, its nucleus,
atmosphere, and tail. M. Faye, taking up Kepler's theory of
an actual repulsive force inherent in the solar rays, discussed
the whole of its consequences and compared them with the
facts observed. At his suggestion M. Roche included this
action also in his analysis. How M. Faye has defined the
force in question we shall see further on ; we will restrict
385 C C
THE WORLD OF COMETS.
ourselves to stating that, under the influence of this force,
the figure of the surfaces of equilibrium ceases to be symme-
trical with respect to the central nucleus. The free surface,
convex and slightly flattened towards the sun, is terminated at
the opposite extremity by a conical apex. The inner surfaces
of equilibrium envelop the nucleus on all sides ; but the
Fig. 65.— Development of cometary tails, on the hypothesis of an intense repulsive force.
M. Eoche's theory.
outer surfaces closed towards the sun are open on the oppo-
site side, in the vicinity of the conical point A (fig. 65). By
this single opening behind the nucleus the excess of cometary
matter makes its escape. Hence we find regularly superim-
posed layers towards the sun, in the form of envelopes ; on
the other side these layers are traversed and as it were broken
Fig. 66. — Development of cometary tails on the hypothesis of a feeble repulsive force.
M. Roche's theory.
by emissions from the summit J., which is the origin of a tail
opposite the sun ; such is the aspect a comet would present in
the new conditions to which we have supposed it subjected.
Also the figure varies, if the intensity of the repulsive
force varies with the nature of the particles upon which
it acts. Hence the different forms which may co-exist
386
THEORY OF COMETARY PHENOMENA.
in the same comet. The three theoretical figures that we have
given correspond, the first (fig. 64) to the absence of a re-
pulsive force, the second (fig. 65) to a repulsive force acting
upon the whole of the fluid with great intensity. The third
(fig. 66) supposes the force in question to be extremely feeble.
The tail directed towards the sun exists then but as a very
slightly elongated aigrette, which is speedily drifted back to unite
with the opposite tail.
M. Roche then examines in detail several of the observed
facts that we have described, and shows that in his theory
they find a logical interpretation: the successive envelopes
formed by the vapours raised from the nucleus by the influence
of the solar heat, the formation of aigrettes towards the sun, and
the drifting back of the emitted matter, the general form of
the tail, more brilliant as a rule on its outer edge, where the
repelled matter is accumulated, the dark sector behind the
nucleus, &c.
There are, doubtless, in any particular comet complica-
tions of aspect which require special study. These may arise
from the peculiar constitution of the comet itself, or quite
as frequently from changes in the relative position of the
observed comet and the earth, in which case they are only
effects due to perspective.
The theory of M. Roche has received the adhesion of several
French and foreign astronomers. And in our opinion it has
one great merit : it is more than a simple hypothesis, because
it rests upon two undisputed principles in astronomy, as it
only depends upon the action of gravitation and solar heat;
and when the new force — the repulsive force — is introduced no
hypothesis is made with regard to its origin and nature. The
sole hypotheses assumed by M. Roche consist (1) in the re-
pulsive force, which he regards as directly proportional to the
density of the matter submitted to its influence and inversely
387 c o 2
THE WORLD OF COMETS.
to the square of the radius vector, arid (2) in a fact con-
cerning which all contemporary astronomers appear to be
agreed, viz. the extreme tenuity of the cometary matter in
those portions of the nebulosity which are detached from the
nucleus. ' This tenuity,' he observes, * is so great that it ex-
ceeds anything that we can imagine, and renders the nebulosity
of a comet comparable to the medium which occupies the vacuum
of an air-pump. The repulsion which the comet appears to
experience on the part of the sun would, therefore, seem to be a
consequence of that singular condition of matter concerning
which physical science does not as yet possess any certain
data.'*
* 'From another point of view,' adds M. Roche, ' the study of a substance
reduced to this state of extreme dilatation would be of not less importance.' In
fact, as M. Radau has judiciously remarked, ' the matter of the tails of comets is so
disseminated and rarefied that we are compelled to renounce the idea of express-
ing their density in figures. This extreme division of the matter is not so
extraordinary as might be thought. It is comparable to the density of cosmical
matter, the concentration of which, according to Laplace, has given birth to the
planets and their satellites. Let us suppose the earth's radius increased till it
becomes equal to the distance of the moon ; it would then be so rarefied that it
would become fifty times less dense than ordinary air ; and if the sun were ex-
panded till its radius was equal to that of the terrestrial orbit, its density would
not be more than sixteen-millionths of that of atmospheric air. It is then a
medium similar in its extreme rarity to cometary tails, that has constituted the
atmosphere of the sun during the period of its condensation. The as yet unknown
properties of this medium will no doubt ultimately throw light upon the primi-
tive condition of the solar system.'
It is not only to the terrestrial orbit that we should suppose the radius
of the sun or of the planets extended in order to reconstitute the primitive
solar nebula ; it should be extended at least to the orbit of Neptune. By
supposing a homogeneous sphere of this radius filled with the matter of the sun, its
density as compared with that of water would be OOOO 000 COO 005 29, which is a
little more than five billionths of the actual density of water. This is a density-
two hundred and fifty millions of times less than that of atmospheric air.
388
SECTION VII.
THE REPULSIVE FORCE A REAL PHYSICAL FORCE.
Theory of M. Faye — Rigorous definition of the repulsion inherent in the solar rays — •
Its intensity varies with the surfaces of the two bodies ; it decreases inversely as
the square of the distance — It is not propagated instantaneously — Discussion and
accordance of the facts — Experiments in support of a repulsive force.
IT was at the suggestion of M. Faye, as we have seen, that
M. Roche introduced into his analytical researches upon come-
tary phenomena the hypothesis of a repulsive force which has,
in fact, led to results more in conformity with what is observed.
It should be remarked, however, that M. Roche has considered
the matter rather from the point of view of a mathematician
than of a physical astronomer ; whilst, on the contrary, the
physical bearing of the problem has more especially occupied
the attention of M. Faye. This astronomer, after passing in
review the different theories we have mentioned, and rigorously
comparing their conclusions with the facts of recorded observa-
tions, in short, after the most exhaustive discussion, has finally
decided in favour of an actual repulsive force inherent in the
solar rays. This is the base of the theory known as Kepler's
theory, and which has been distinguished by the adhesion of
Euler and Laplace.
At the time when M. Faye made known his views, two
great comets — that of Donati (1858) and that of 1861 — -had
389
THE WORLD OF COMETS.
recently appeared. Both comets had been subjected to careful
telescopic scrutiny, and it was necessary to explain the physical
phenomena which had been daily followed in their details by
observers in Europe and America, and also to account for a
phenomenon of another kind, but equally important, viz. the
accelerated movement of the comets of Encke and Faye.
Encke, as we shall see, was in favour of the hypothesis of a
resisting medium, and regarded it as the cause of the known
acceleration of the above-mentioned comets. Newton likewise,
as we have seen, attributed the formation of cometary tails to
the existence of an interplanetary medium. Here, then, is a
connexion between two very different classes of phenomena.
M. Faye, on the hypothesis of a repulsive force, proceeds to
examine the cause of the formation and development of aigrettes
and tails, and the accelerated movement of the comets above
referred to.
Let us see, in the first place, how M. Faye defines the
repulsive force. This is an important point, concerning which
the partisans of this theory had hitherto neglected to be ex-
plicit ; there was supposed to be an impulsion of the solar rays,
and that was all. M. Faye's words are: —
' A repulsive force having its origin in heat. By means of
it heat produces mechanical effects. It depends upon the sur-
face and not upon the mass of the incandescent body. The ac-
tion upon a body is proportional to the surface of the body, and
not to its mass. It is not propagated instantaneously, like the
attractive force of Newton. Nor does it act through intervening
matter, like attraction. It is provisionally assumed that its
intensity decreases inversely as the square of the distance, and
that its velocity of propagation is the same as that of rays of
light or heat.'
Now, the existence of such a force being admitted, how
does M. Faye deduce from it the theory of cometary pheno-
390
THE REPULSIVE FORCE A REAL PHYSICAL FORCE.
mena ? How does he explain by it the formation of tails,
simple or multiple, their curvature, their direction, the develop-
ment of luminous or dark sectors, the disengagement of enve-
lopes more or less parabolic ? Upon all these points M. Faye
gives the following explanations, which he finds confirmed
point by point by the observations of the most brilliant comets
which have recently appeared : —
1 The action of the repulsive force upon a body in motion
about the sun does not coincide with the radius vector, but is
always exerted in the plane of the orbit, so that the figure
which it tends to impress upon a body originally spherical,
such as that of a comet very remote from the sun, will be sym-
metrical with respect to this plane ; nor can this result be
changed either by the sun's attraction or by that of the nucleus,
or by the progress of the deformation itself. In the second
place, the action of this force being in proportion to the surface,
the effects produced depend upon the density of the matter
of which the comet is composed ; it follows, then, that, except
in the plainly exceptional event of these materials being com-
pletely homogeneous, it must give rise to the formation of
several tails, resulting from a sort of purely mechanical selec-
tion on the part of the repulsive force. But the axes of these
multiple tails, which are longer in proportion as their curvatures
are less, will always be situated in the plane of the orbit, as in
the case of a single tail.
' According to the mechanical generation of these append-
ages, the matter of which is in a state of division, tenuity, and
molecular independence whereof it is difficult to form an idea,
each tail, in its regular portion, exhibits a simple curvature
behind the line of motion of the nucleus At its origin each
of these tails is tangential to the radius vector, or rather is
inclined to it at a small angle.
' With respect to the special form of any particular tail, it
391
THE WORLD OF COMETS.
must be regarded as the envelope of matters of the same density
which successively abandon the head of the comet, under the
triple influence of the repulsive force, the power of solar attrac-
tion, and the general velocity, to which we must add, as Bessel
has done, the small velocity of the nucleal emission. If we
consider at a given moment the whole of the molecules thus
driven from the narrow sphere of attraction of the comet, they
will be found principally distributed over the circumference of
a nearly circular section of the nebulosity ; and if we follow the
same series of particles for the next few moments, we shall see
that, as a consequence of their motions in their independent
trajectories, the nature of which can be assigned, they will
occupy constantly increasing areas, the section continually
lengthening in the plane of the orbit, while the transverse
diameter increases in much less proportion. The tail of the
comet, therefore, will be principally displayed in the plane of
the orbitr more especially tails which are very much curved.
But should they be viewed edgewise, they will appear straight,
under the form of a narrow band, equally denned on the two
edges, and more brilliant at the edges than in the middle. The
two edges will be nearly parallel, or at all events but slightly
divergent, unless the observer should be situated very near to
a portion of the tail... Should there be several tails, they will
appear projected one upon another, while the earth is crossing
the plane of the comet's orbit, and as they are very far from
being opaque, the narrowest of the tails will be seen defined in
the midst of the largest, or that which is nearest to the observer.
It is evident, therefore, that before they can be distinguished
one from another the earth must have passed by a very con-
siderable distance the plane of the comet's orbit.'
The above is M. Faye's explanation of the origin and de-
velopment of tails, as well as of the varied appearances obser-
vable in cometary appendages. As a whole, this theory is
392
THE REPULSIVE FORCE A REAL PHYSICAL FORCE.
certainly satisfactory, but we cannot affirm, in presence of the
numerous and complex facts which we have described, that it
is quite complete or free from objection. For example, we do
not see very clearly how M. Faye would explain the appearance
of the multiple fan-shaped tail presented by the great comet of
1861, on June 30, the day when the earth was situated exactly
in the plane of the orbit. In this situation the tails of the
comet should have been seen projected upon each other, as de-
scribed. But these are minor difficulties, arising, doubtless,
from the real complexity of the phenomena, further enhanced
by the effects of perspective.
As regards nucleal emissions, the sectors and luminous
envelopes, &c., their formation is considered to be wholly
attributable to the forces of attraction and the increasing influ-
ence of the solar calorific radiations. It is here that M. Faye
refers to M. Roche, and considers the theoretical diagrams given
by the latter as the most faithful representations possible of the
real phenomena.
' Thus,' observes M. Faye, in conclusion, ' the figure of a
comet, and the more extended portion of the tail, are the result
of a purely mechanical action of two forces : the Newtonian
attraction and the repulsion due to heat. The attraction is
exercised by the respective masses of the sun and the comet,
the repulsion by the incandescent surface of the sun ; but it is
further necessary to take into consideration the repulsive force
which the heat belonging to the comet, or rather that which it
receives in approaching the sun, develops amongst its molecules.
From this cause arises an expansion more or less analogous
to that of terrestrial bodies when brought to a gaseous state,
an expansion which occurs in the phenomenon of the double
nucleal emission. It is, thus, this expansion which enables the
solar repulsion to take effect, and which, dilating the matter
of the nucleus more and more, renders it of extreme tenuity, as
393
THE WOKLD OF COMETS.
in the already mentioned case of the envelopes of Donati's comet.
The question is, therefore, very simple in principle, notwith-
standing the enormous complexity of the phenomena involved;
and as in the universe there is a relation between all things,
it will be perceived more and more that in the sidereal universe
as well as in the terrestrial domain there exist many other mani-
festations of this repulsive force, the effect of which comets
present to us upon so gigantic a scale.'
However well these deductions of an able theory may accord
with each other and with the fundamental principle, they none
the less rest upon an hypothesis : that of a repulsive force in-
herent in the solar radiations. The theoiy of M. Faye, therefore,
stood in need of the decisive and indispensable sanction that ex-
periment alone can give. This was fully appreciated from the
beginning by M. Faye, who, in 1861, in order to obtain the
confirmation which the former experiments of Bennet had
seemed to promise, made, in conjunction with M. Iluhmkorff, a
series of experiments upon the action which metallic plates,
when heated to a state of incandescence, exercise in vacuo upon
the stratification of the induction spark. ' In all these experi-
ments,' says he, ' the repulsive action of the incandescent surface
was very decided, but I was more especially struck with it in
the case of arsenic and sulphur.' The influence of the heat is
manifest, if it be true that the repulsion increased with the
temperatures of the plates heated to incandescence. The influence
of the density is not less, if the indications of the repulsion were
weak, in proportion to the amount of air admitted within the
globes that were used in the experiments.
The direct verification of the solar repulsion, according to
M. Faye, is impossible on the surface of the earth, ' as in all pro-
bability it exhausts itself upon the upper strata of our atmo-
sphere.' If this be the case, there is no reason, it is clear, why
the earth and all the planets that have atmospheres should not
304
THE REPULSIVE FORCE A REAL PHYSICAL FORCE.
be provided with tails after the manner of comets. It remains
to be determined whether they are sufficiently extended to
be visible, but that they must exist is certain, unless the
repulsive force should be here exhausted without producing
its effect.
[* During the last two years the subject of attraction and repulsion as result-
ing from radiation has been the subject of much discussion and investigation, in
consequence of the experiments of Mr. Crookes and the invention by him of his
radiometer. As the question of whether the solar emanations are accompanied
by a repulsive action is one of the highest importance in regard to the motions
of comets and the explanation of their tails, and as this is a matter which has
been recently the object of the most searching and thorough examination by
means of instruments of extraordinary delicacy, I think it desirable to give a brief
account of what has been effected.
I commence by giving in Mr. Crookes's own words a short summary of a
historical summary of the investigations prior to 1873: —
' The Rev. A. Bennet recorded the fact that a light substance delicately
suspended in air was attracted by warm bodies: this he ascribed to air-currents.
When light was focused, by means of a lens, on one end of a delicately suspended
arm, either in air or in an exhausted receiver, no motion could be perceived dis-
tinguishable from the effects of heat.
' Laplace spoke of the repulsive force of heat. Libri attributed the move-
ment of a drop of liquid along a wire heated at one end, to the repulsive force of
heat, but Baden Powell did not succeed in obtaining evidence of repulsion
by heat from this experiment.
' Fresnel described an experiment by which concentrated solar light and heat
caused repulsion between one delicately suspended and one fixed disk. The
experiment was tried in air of different densities; but contradictory results were
obtained under apparently similar circumstances at different times, and the
experiments were not proceeded with.
' Saigey described experiments which appeared to prove that a marked
attraction existed between bodies of different temperatures.
' Forbes, in a discussion and repetition of Trevelyan's experiment, came to
the conclusion that there was a repulsive action exercised in the transmission
of heat from one body into another which had a less power of conducting it.
' Baden Powell, repeating Fresnel's experiment, explained the results other-
wise than as due to repulsion by heat. By observing the descent of the tints of
Newton's rings between glass plates when heat was applied, Baden Powell showed
that the interval between the plates increased, and attributed this to a repulsive
action of heat.
' Faye introduced the hypothesis of a repulsive force of heat to account for
certain astronomical phenomena. He described an experiment to show that
£95
r THE WORLD OF COMETS.
heat produced repulsion in the luminous arc given by an induction-coil in
rarefied air.'
Mr. Crookes's own experiments showed that a heavy metallic mass when
brought near a delicately suspended light ball attracts or repels it under the
following circumstances : —
I. When the ball is in air of ordinary density —
(a) If the mass is colder than the ball it repels the ball ;
(b) If the mass is hotter than the ball it attracts the ball.
II. When the ball is in a vacuum —
(a) If the mass is colder than the ball it attracts the ball ;
( b) If the mass is hotter than the ball it repels the ball.
And in an experiment in which the rays of the sun, and then the different por-
tions of the solar spectrum, were projected on to a delicately suspended pith-ball
balance, he found that in vacuo the repulsion was so strong as to cause danger
to the apparatus, and resembled that which would be produced by the physical
impact of a material body.
The application of these facts to the question of a solar repulsive action is
obvious ; and Mr. Crookes himself, after discussing the explanations which may
be given of the phenomena and showing that they cannot be due to air-currents,
thus referred to the evidences of a repulsive action of heat and attractive action
of cold in nature. ' In that portion of the sun's radiation which is called heat we
have the radial repulsive force possessing successive propagation, required to
explain the phenomena of comets and the shape and changes of the nebula?.
To compare small things with great — to argue from pieces of straw up to
heavenly bodies — it is not improbable that the attraction now shown to exist
between a cold and a warm body will equally prevail, when, for a temperature
of melting ice is substituted the cold of space, for a pith-ball a celestial sphere,
and for an artificial vacuum a stellar void.'
All this is taken from Mr. Crookes's abstract of his paper (Proc. Roy. Soc.,
vol. xxii. pp. 37-41). The paper itself was printed in the Philosophical Trans-
actions, vol. clxiv. pp. 501-527.
By these experiments Mr. Crookes was led to examine more fully the action
of radiation upon black and white surfaces. He found that at the highest
exhaustion heat appeared to act almost equally on white and on lampblacked
pith, repelling them in about the same degree, but that the action of luminous
rays was different. These were found to repel the black surface more energeti-
cally than the white surface. Taking advantage of this fact, Mr. Crookes was
led to invent the instrument now so well-known as the radiometer or ' light-
mill.' It consists of four arms of very fine glass, supported in the centre by a
needle-point, so that it is capable of revolving horizontally. To the extremity
of each arm is fastened a thin disk of pith, lampblacked on one side, the black
surfaces all facing the same way. The arms and disks are delicately balanced,
so as to revolve with the slightest impetus. The whole is enclosed in a glass
globe, which is then exhausted to the highest attainable point and hermetically
396
REPULSIVE FORCE A REAL PHYSICAL FORCE.
sealed. This instrument revolves under the influence of radiation, the rapidity
of revolution being in proportion to the intensity of the incident rays.
The speed with which a sensitive radiometer will revolve in full sunshine is
almost incredible, the number of revolutions per second being several hundreds.
One candle will make the arms spin round forty times a second. Mr. Crookes found
that the action of dark heat (as, e.g. from boiling water) was to rep«l each surface
equally, and the movement of the radiometer is therefore arrested if a flask of
boiling water is brought near it. The same effect is produced by ice.
In a brief notice of his subsequent experiments read before the Royal
Society on June 15 of the present year, Mr. Crookes attributes the repulsion
caused by radiation as shown by the radiometer to the action of the residual gas,
i.e. to the very small amount of gas (the gas employed was generally dry atmo-
spheric air, but the effect only differed in degree with other gases) still remaining
in the almost perfect vacuum. ' In the early days,' says Mr. Crookes, ' of this
research, when it was found that no movement took place until the vacuum was
so good as to be almost beyond the powers of an ordinary air-pump to produce,
and that as the vacuum got more and more nearly absolute, so the force increased
in power, it was justifiable to assume that the action would still take place
when the minute trace of residual gas which theoretical reasoning proved to be
present was removed.
' The first and most obvious explanation, therefore, was that the repulsive
force was directly due to radiation. Further consideration, however, showed
that the very best vacuum which I had succeeded in producing might contain
enough matter to offer considerable resistance to motion. I have already pointed
out that in some experiments where the rarefaction was pushed to a very high
point the torsion-beam appeared to be swinging in a viscous fluid ; and this
at once led me to think that the repulsion caused by radiation was indirectly
due to a difference of thermometric heat between the black and white surfaces of
the moving body, and that it might be due to a secondary action on the
residual gas.'
Mr. Crookes having contrived an apparatus by means of which the viscosity
of the residual internal gas, as well as the force of the radiation, could be mea-
sured, found that up to an exhaustion at which the gauge and the barometer
were sensibly level there was not much variation in the viscosity of the internal
gas, and that on continuing to exhaust, the force of radiation commenced to be
apparent, the viscosity remaining about the same. The viscosity next commenced
to diminish, the force of radiation increasing. After long-continued exhaustion
the force of radiation approached a maximum, but the viscosity began to fall off:
at a still higher exhaustion the force of repulsion diminished. In a radiometer
exhausted to a very high degree of sensitiveness the viscosity of the residual
gas- is almost as great as if it were at the atmospheric pressure. Mr. Crookes
concludes as follows : ' The evidence afforded by the experiments of which this
is a brief abstract is to my mind so strong as almost to amount to conviction,
that the repulsion resulting from radiation is due to an action of thermometric
heat between the surface of the moving body and the case of the instrument,
397
THE WORLD OF COMETS.
through the intervention of the residual gas. This explanation of its action is
in accordance with recent speculations as to the ultimate constitution of matter
and the dynamical theory of gases.' It will thus be seen that although the radio-
meter at one time seemed to afford experimental evidence of the direct repulsive
action of the light-rays, Mr. Crookes after a remarkable series of experiments
extending over four years has come to the conclusion that at all events the repul-
sion is only an indirect effect of the action of light ; so that the evidence in favour
of a real solar repulsive force, after being submitted to a very severe test, has
been found wanting.
It is to be noticed that although Mr. Crookes seems to have in his first paper
inclined to the belief that his experiments tended to establish a direct action of
radiation, he has throughout refrained from adopting this as a theory, and the
final conclusion quoted above is I believe the only explanation he has at any
time offered of the action of the radiometer.
The existence of a solar repulsive action has not, I think, ever found much
favour with mathematical physicists, and recent investigations do not seem to
have afforded experimental evidence in favour of it ; but at present, while the
details of the chief experiments of Mr. Crookes remain still unpublished, it would
be premature to attempt to decide under what circumstances repulsion may result
directly or indirectly from radiation. In any case it now may be considered as
certain that the matter is one that can be satisfactorily determined by experi-
ments in the laboratory, and in consequence of the interest that has been excited
there is little doubt that before very long much more will be known upon the
subject than at present.
Abstracts of Mr. Crookes's papers have been published in the Proceedings of
the Royal Society, vol. xxii. p. 37, vol. xxiii. p. 373, vol. xxiv. pp. 276 and 279,
and vol. xxv. p. 136. Two papers only have as yet been printed in extenso in
the Philosophical Transactions, (vol. clxiv. p. 501 and vol. clxv. p. 519.)
Mr. Bennet's paper was published in the Philosophical Transactions for 1792.
I may add in conclusion, that in my own opinion the solar repulsive force
seems to me still to be merely a hypothesis, and I cannot feel that any explana-
tion of cometary phenomena that is dependent upon it is satisfactory. — ED.] ^
398
SECTION VIII.
THEORY OF THE ACTINIC ACTION OF THE SOLAR RAYS.
Experiments and hypotheses of Tyndall — Originality of his theory: objections and
omissions — Is this theory incompatible with that of a repulsive force ?
A NEW theory of cometary phenomena which has been proposed
by Professor Tyndall, one of the most distinguished of contem-
porary physicists, in our opinion merits special attention. In
the first place, because we believe it to be altogether new and
original ; and, in the second place, because it is derived, not
from a priori conceptions, like so many other theories in astro-
nomy and physics, but from accurate experiments and their
interpretation.
The study of the action of radiations upon very rarefied
media of gaseous matter first led Professor Tyndall to consider
the mode of production of the phenomena presented by the
heads and tails of comets. Of the undulations proceeding from
any luminous source, such as the sun, some have a purely
calorific action ; these are those which have the greatest am-
plitude or are least refrangible; the undulations which consti-
tute or produce light come next in the order of length of wave
or refrangibility ; the shortest waves are those which manifest
themselves exclusively by chemical action. We now proceed
to explain Professor Tyndall's views on the subject of these
modifications, and his manner of accounting for the fact that the
399
THE WORLD OF COMETS.
rays of shortest wave-length are endowed with the property of
acting upon chemical substances, decomposing them and sepa-
rating the atoms of which their molecules are composed, whilst
the larger and mechanically more powerful waves are, on the
contrary, ineffectual to perform any such decomposition.
' Whence, then, the power of these smaller waves to unlock
the bond of chemical union? If it be not a result of their
strength, it must be, as in the case of vision, a result of their
periods of recurrence. But how are we to figure this action?
I should say thus : the shock of a single wave produces no more
than an infinitesimal effect upon an atom or a molecule. To
produce a larger effect the motion must accumulate ; and for
wave-impulses to accumulate they must arrive in periods
identical with the periods of vibration of the atoms on which
they impinge. In this case each successive wave finds the
atom in a position which enables that wave to add its shock to
the sum of the shocks of its predecessors. The effect is me-
chanically the same as that due to the timed impulses of a boy
upon a swing. The single tick of a clock has no appreciable
effect upon the unvibrating and equally long pendulum of a
distant clock ; but a succession of ticks, each of which adds, at
the proper moment, its infinitesimal push to the sum of the
pushes preceding it, will, as a matter of fact, set the second
clock going.'
After having thus explained the chemical action of light,
Professor Tyndall proceeds to study its action upon the vapours
of different volatile substances, sometimes employing a beam
of electric light, and at other times the solar light. He fills a
tube of certain length with a mixture of air and the vapour of
nitrite of amyl, of nitrate of butyle, or of iodide of allyl, after
having taken the requisite precautions for the exclusion of all
foreign matters, and more especially of particles floating in the
air — dust, organic germs, mineral matters, &c. When thus
400
THEORY OF THE ACTINIC ACTION OF THE SOLAR RAYS.
filled the tube remains dark, and the mixture it contains
is absolutely invisible. But should a luminous beam of light,
such as that given by the flame of a lamp, be rendered conver-
gent by a lens and allowed to fall upon the interior of the tube,
the following will be observed : the space for an instant after
the introduction of the beam will remain dark; but this brief
moment passed, a white luminous cloud will be seen to invade
that portion of the tube occupied by the beam of light. How
has this change been effected ? The action of the waves has
decomposed the nitrite of amyl and precipitated a rain of par-
ticles which from that moment are capable of reflecting and
diffusing in all directions the light of the beam. ' This experi-
ment,' says Tyndall, ' illustrates the fact, that however intense
a beam of light may be, it remains invisible until it has some-
thing to shine upon. Space, although traversed by the rays
from all suns and all stars, is itself unseen. Not even the ether
which fills space, and whose motions are the light of the uni-
verse, is itself visible.'
We may see by this last remark the capital objection which
forces astronomers to reject the theory of Cardan, according to
which the tails of comets are simply the effect of refraction.
This theory we have already mentioned.
It is to be remarked that the end of the experimental tube
most distant from the lamp is free from cloud. Now, the
nitrite of amyl vapour is there also, but it is unaffected by the
powerful beam passing through it. Why ? Because the very
small portion of the beam competent to decompose the vapour
is quite exhausted by its work in the frontal portions of the
tube ; it is the longer waves that continue their course ; but
these waves are powerless to produce a chemical decom-
position. Thus can the able physicist find in the detail of
facts the confirmation of his ingenious hypotheses. But let
us now proceed to the theory of cometary phenomena.
401 D D
THE WORLD OF COMETS.
The substance of this theory has been embodied by Pro-
fessor Tyndall in the seven following propositions, which
we will reproduce in the author's own words :—
' 1. The theory is, that a comet is composed of vapour de-
composable by the solar light, the visible head and tail being
an actinic cloud resulting from such decomposition ; the tex-
ture of actinic clouds is demonstrably that of a comet.
' 2. The tail, according to this theory, is not projected
matter, but matter precipitated on the solar beams traversing
the cometary atmosphere. It can be proved by experiment
that this precipitation may occur either with comparative slow-
ness along the beam, or that it may be practically momentary
throughout the entire length of the beam. The amazing
rapidity of the development of the tail would be thus
accounted for without invoking the incredible motion of
translation hitherto assumed.
' 3. As the comet wheels round its perihelion, the tail is
not composed throughout of the same matter, but of new
matter precipitated 011 the solar beams, which cross the
cometary atmosphere in new directions. The enormous
whirling of the tail is thus accounted for without invoking a
motion of translation.
' 4. The tail is always turned from the sun, for this
reason : two antagonistic powers are brought to bear upon the
cometary vapour — the one an actinic power, tending to effect
precipitation ; the other a calorific power, tending to effect
vaporisation. Where the former prevails, we have the cometary
cloud ; where the latter prevails, we have the transparent
cometary vapour. As a matter of fact, the sun emits the
two agents here invoked. There is nothing whatever hypo-
thetical in the assumption of their existence. That precipi-
tation should occur behind the head of the comet, or in the
space occupied by the head's shadow, it is only necessary to
402
THEORY OF THE ACTINIC ACTION OF THE SOLAR RAYS.
assume that the sun's calorific rays are absorbed more copiously
by the head and nucleus than the actinic rays. This
augments the relative superiority of the actinic rays behind
the head and nucleus, and enables them to bring down the
cloud which constitutes the comet's tail.
' 5. The old tail, as it ceases to be screened by the nucleus,
is dissipated by the solar heat ; but its dissipation is not in-
stantaneous. The tail leans towards that portion of space last
quitted by the comet — a general fact of observation being thus
accounted for.
'6. In the struggle for mastery of the two classes of rays
a temporary advantage, owing to variations of density or some
other cause, may be gained by the actinic rays, even in parts
of the cometary atmosphere which are unscreened by the
nucleus. Occasional lateral streamers, and the apparent
emission of feeble tails towards the sun, would be thus ac-
counted for.
' 7. The shrinking of the head in the vicinity of the sun is
caused by the breaking against it of the calorific waves, which
dissipate its attenuated fringe and cause its apparent con-
traction.'
This very brief exposition of an hypothesis which might
be termed the physico-chemical theory is taken from the new
edition of Tyndall's work upon Heat. It is unaccompanied by
explanation or commentary of any kind, and to us at least seems
to be wanting in completeness, and to contain some obscurities
which we shall briefly notice, in the form of questions and
objections, and on the subject of which we should be glad to
receive additional elucidation from the author.
Professor Tyndall defines comets without making mention
of the nucleus. Comets, for him, would appear to be simple
masses of vapour rendered visible by the actinic action of the
solar rays. Further on, nevertheless, he considers the nucleus
403 D D 2
THE WORLD OF COMETS.
as endowed with the property of absorbing the calorific waves,
whilst the efficacy of the chemical waves is in no respect im-
paired. In those cornels where observation has proved that a
nucleus exists, is that nucleus solid or liquid, or a simple
gaseous mass of greater density than the other portions of the
comet?
He considers the chemical and calorific rays as antagonistic
powers. Nevertheless they are regarded by all physicists as
undulatory movements differing in no essential respect from
each other, or in other words differing only in amplitude and
length of period. In what respect, then, are the calorific and
chemical waves opposed ?
What is this dissipation by the solar heat of the tail no
longer screened by the nucleus ? Is it the effect of a repul-
sive force inherent in the calorific rays which are no longer
absorbed by the nucleus? Are the particles precipitated by
the decomposing action of the actinic rays re-composed so as
to resume their original state of transparency?
Professor Tyndall makes no mention of the curvature of
the tail, of that disposition which causes it to be displayed in
the plane of the orbit, or of the production of multiple tails.
Do the accidental lateral currents of which he speaks afford
a sufficient reply to this last question?
Finally, according to this theory, it follows that comets are
agglomerations of matter of extreme tenuity, certain parts only
of which are rendered visible by the solar action. The pre-
cipitation takes place in the interior of the mass, in a deter-
minate but continually varying direction, so that the mass
would have to be considered as having in every direction a
diameter equal to the enormous length of the comet's tail.
These spheres of vapour, therefore, millions of miles in
diameter, thus travel in known orbits in the midst of the
interplanetary spaces. Is it gravitation towards the nucleus
404
THEORY OF THE ACTINIC ACTION OF THE SOLAR RAYS.
which thus keeps together the constituent molecules of these
attenuated masses ; or if not, if the vapours thus formed are
incessantly abandoned in space, how are they incessantly re-
placed ? Are they an emission proper to the nucleus, or the
effect of a repulsive action of the solar heat ? If Professor
Tyndall should admit this last hypothesis, of what use
would l>e the actinic action of the solar rays in explaining the
development of the tail? His theory would in that case be
grafted upon the theory of M. Faye, and would have no other
raison d'etre than to explain the visibility of a matter so
attenuated as that of which the tails of cornets are composed.
In our opinion all these questions require elucidation ; but
they are rather questions than objections. We must not for-
get that the forces called into play in phenomena of this
kind — that is to say, the recognised forces — are gravitation and
the ethereal radiations. It has been supposed by some that
gravitation itself is due to the waves of ether ; these last
are revealed to us by their triple manifestations — calorific,
luminous, and chemical. Professor Tyndall invokes chemical
action for the purpose of explaining cometary phenomena;
M. Roche and M. Faye appeal to gravitation and heat. It
remains to be seen whether the repulsive force may not be
explained as a component of the solar radiations. If so, by
far the greatest obstacle to the reconciliation of these different
theories would be removed, and the cause of all those curious
and diverse phenomena, the movements of comets, their per-
turbations, as well as their physical transformations, whether
apparent or real, would be reduced to the one principle which
Lame* believed to be the universal connecting link between
all the phenomena, viz. the undulatory movement of the ether.
40o
SECTION IX.
COMETS AND THE RESISTANCE OF THE ETHER.
Accelerated motion of Encke's comet ; its periods continually diminish — It describes a
spiral, and will ultimately fall into the sun — Hypothesis of a resisting medium ;
how does the resistance of a medium increase the rapidity of motion ? — The nature
of this supposed medium, according to Arago, Encke, and Plana — Objections of
M. Faye; the acceleration of motion explained by the tangential component
of the repulsive force.
IN our account of the periodic comet of Encke we gave,
together with the dates of its successive apparitions, the
durations of the revolutions comprised between these dates.
If the reader will turn back to the table on p. Ill he will
readily perceive that these durations are unequal, and that the
period is continually decreasing, and has suffered a diminution
of a little more than two days, or exactly of 2*06 days. As
the table includes twenty-two revolutions of the comet it is at
most a diminution in each revolution of two hours twenty-
two minutes, a quantity small in itself, but which, incessantly
accumulating, is capable of producing changes of very great
importance in the course of time.
The discovery of this acceleration is due to the astronomer
whose name the comet bears. Since 1824 Encke had re-
marked the diminution of the period, and he was unable to
account for the result observed, even by admitting very con-
siderable errors in the masses of the planets, whose disturbing
406
COMETS AND THE RESISTANCE OF THE ETHER.
influence upon the comet he had himself calculated with the
utmost care ; but, by assuming the existence of a resisting
medium, Encke found that the major axis of the orbit would
decrease as well as the eccentricity, and that the mean motion
would increase, while the inclination and the longitude of the
node would remain unchanged. As this agreed with obser-
vation he was led to attribute the acceleration of the motion
of the comet to the resistance of a medium now generally
spoken of as the resistance of the ether.
, Encke continued his researches on the subject on each
return of the comet ; he calculated with care, taking into
account the disturbing influence of the planets, the epoch of
the perihelion passage of the comet ; lastly, he published, in
1858, the memoir from which we have extracted the table,
which exhibits in a striking manner the acceleration of its
movement.
In order to explain this diminution of period Encke, as we
have already seen, regards the interplanetary spaces, not as" a
vacuum, as assumed by Newton and astronomers of his
school,* but as filled with a medium of suificient density to
oppose to the movements of bodies circulating therein a re-
sistance capable of producing in the course of time modifi-
cations in their orbits. The mean augmentation of the motion
of the comet, which has been found by observation, arises,
according to him, from a tangential force acting in a direction
opposite to that of the comet's motion, ' which accords entirely,'
he observes, 'and in the most simple manner, with the hypo-
thesis of a resisting medium in the universe. Proofs of the
* Newton, though regarding the interplanetary space as a vacuum, explains
nevertheless, as we have seen, the formation of tails by an ascensional movement
of the cometary particles in the midst of a ponderable medium which surrounds
the sun to a certain distance, and whose density increases in proportion as this
distance decreases. It is really, therefore, the hypothesis of Newton which in thia
case has been adopted by Encke.
407
THE WORLD OF COMETS.
existence of such a medium appear so evident that there can no
longer be any doubt about the matter.' We shall see, however,
that the evidence is not considered satisfactory by all astro-
nomers; and, in particular, M. Faye has raised objections
which it is difficult to ignore. But, from whatever cause may
arise the acceleration of Encke's comet, and whatever may be
the physical nature of the force in question, it has always been
regarded by mathematicians and astronomers as a resistance
applied to the comet, and exerted in a direction opposite to
that of its motion.
On this point we must enter into some details, in order to
explain what always seems strange to those persons who have
but a slight knowledge of the principles of mechanics, and of
celestial mechanics in particular. Such persons find it difficult
to understand that a resistance experienced by a body in
motion, describing a certain curve around a centre, should
produce an acceleration. ; it appears to them that the reason-
ing is false, and that a retardation must be the necessary
result of such a resistance. They would be right if the
body in question were moving in a determinate and in-
variable line from which it could never swerve. A railway-
train, for example, moving against a strong wind expe-
riences a resistance which reduces its speed. This is not the
case with a comet or any other body which, animated by a
determinate velocity, and free to take any direction whatever
in space, necessarily follows a course dependent, on the one
hand, upon the velocity it has at any instant, and, on the
other, upon the force with which the sun attracts it.
Now, if the velocity of the comet undergo a diminution,
such as would be caused by a resisting medium, then, as the
force to the sun remains the same, the comet would, as it were,
be pulled in towards the sun, so that instead of describing CC',
it would in the same time describe CC". But, by Kepler's
408
COMETS AND THE RESISTANCE OF THE ETHER.
third law, which Newton proved by means of the principle
of gravitation, there is a fixed relation between the major
axis of an orbit and the periodic time ;* so that if the mean
distance from the sun be diminished the time of revolution
must also be diminished.
The effect, therefore, of a resisting medium upon a comet
would be to dimmish the size of its orbit, and consequently to
shorten its period of revolution. As, moreover, the same
cause would be unceasingly in operation, and even — if we
suppose that the medium gradually increases in density
IS
Fig. 67. — Influence of a resisting medium upon the orbit of a comet.
towards the sun — would act with increasing intensity, the ac-
celeration itself would gradually become more considerable.
The comet, therefore, would describe a curve continually
and steadily approaching the sun; in other words, it would
describe a spiral, and at the end of a certain time would be
precipitated upon the sun itself.
What we have observed of Encke's comet applies equally to
all other comets, and to the various celestial bodies, planets or
satellites, which compose the solar system ; only as regards the
planets, whose masses, or rather densities, are very great in
comparison with those of comets, the effect of this resistance
has been up to the present time imperceptible. It is but a
question of time, however; and, as Arago observes, 'mathe-
matically speaking, if no cause should be discovered which will
compensate for the resistance experienced, it will be certain
* The squares of the periodic times are as the cubes of the major axes. See
note, p. 71.
409
THE WORLD OF COMETS.
that, after a sufficient lapse of time— consisting, perhaps, of
several thousands of millions of years — the earth itself will
be united to the sun.'
But let us leave the planets, which are only remotely con-
cerned in the question, and return to the resisting medium.
What is this medium ? According to Arago it is ether ; ' that
is to say, the ethereal matter which fills the universe, and
whose vibrations constitute light.' Such is not the opinion of
Encke, who considers the resisting medium as a kind of atmos-
phere enveloping the sun on all sides to a certain distance, and
whose density increases inversely as the square of the distance.
The origin of this medium would be either a primitive atmos-
phere of the sun or the debris of atmospheres left in space by
the planetary arid cometary masses. Further, Plana, in a me-
moir upon the subject treated of by Encke, thus expresses him-
self: 'The resisting medium to which the formulae have been
applied is not the imponderable and universal ether which
propagates light, but a kind of atmosphere surrounding the
sun.'
We have already had occasion to speak of objections that
have been made to this hypothetical medium. M. Faye, after
remarking that 'the analysis of those mathematicians who
assume its existence proves that they regard this kind of pon-
derable atmosphere as immovable,' proceeds : ' Now, this im-
mobility is impossible ; no ponderable particle can exist in the
solar system without precipitating itself towards the sun or
circulating about him ; there can be no other alternative.'
M. Faye shows that the" second supposition is alone possible ;
but that if there were a resisting medium circulating about
the sun the effect would be somewhat different. Instead,' he
proceeds, ' of forcing the comet to describe a spiral, so as
to approach the sun, and finally precipitate its mass upon
him, the action of such a medium would chiefly affect the
410
COMETS AND THE RESISTANCE OF THE ETHER.
eccentricity. If this element be sufficiently diminished, the
orbit would become more and more circular, but the major
axis would cease to diminish, and the comet would not be pre-
cipitated upon the sun. In the case of a direct comet, such as
that of Encke, the action of a medium circulating in the same
direction would depend only upon the relative velocity of the
comet and the layers it encounters. There would be alter-
nate periods of acceleration and retardation. The first would
predominate until the orbit became circular ; the influence of
the medium would then cease. A retrograde cornet — Halley's
comet is the sole instance amongst periodic comets — would, on
the contrary, experience a much greater resistance, and the
acceleration would be very considerable.'
This objection relates to an event requiring for its deter-
mination a longer series of observations than we yet possess.
It is very possible that the result may be as indicated by M.
Faye, and that the acceleration of Encke's comet will have a
limit. The hypothesis of a resisting medium appears, then, in
no respect invalidated.
But M. Faye, as we should expect, has sought in his own
theory for an explanation of the observed acceleration. The
repulsive force, by the aid of which he accounts for the pheno^
mena of tails, furnishes him quite naturally with the means.
In fact, the action of this force, as we have seen, is not instan-
taneous; it is propagated with the velocity of rays of light
or heat. A kind of aberration ensues — a deviation in the
direction along which the repulsive force acts. We may,
therefore, conceive of this force as a compound of two parts, viz.
the radial component, CF(Fig. 68), in the direction of the radius
vector, which causes the formation of tails; the tangential
component, Cb, opposite to the direction of motion of the
comet. It is the latter that plays the part of a resisting
medium, and causes, whilst allowing the attraction of the sun
411
THE WORLD OF COMETS.
to preponderate, the diminution of distance from the sun and
the accelerated motion of the comet.
Further, every repulsive force, whatever be its nature, so
long as its velocity of propagation is not infinite, will produce
the same effect and explain in the same manner the acce-
leration of motion. Bessel, in the memoir that we have
referred to, thus expresses himself on this subject : ' The
luminous aigrette of Halley's comet gave it nearly the aspect
of a rocket. Consequently it must have exercised an effect
similar to that which is observed in the movement of rockets.
It is not the centre of gravity of the comet only, but the
centre of gravity of the comet and the aigrette, which describes
Fig. 68. — Radial and tangential components of the repulsive force, according to M. Faye.
a conic section according to the laws of Kepler; the luminous
matter, therefore, which issues from the comet to form
the aigrette must exercise upon the centre of gravity a re-
pulsive action, which, being continuous, produces an accele-
rating force. From the brilliancy of the aigrette, which gives
the apparent proportion of its mass to that of the nucleus, we
may imagine that this disturbing force might very appreciably
change the elliptic movement. In the cornet of 1811 the
delicate researches of M. Argelander appear to indicate a devi-
ation arising from an analogous cause ; the more exact obser-
vations of Halley's comet will allow a closer investigation of
the subject.'
412
COMETS AND THE RESISTANCE OF THE ETHER.
Biot came to a similar conclusion, taking into account the
loss of substance the comet would experience on each of its
revolutions.
Such are the hypotheses that have been proposed, in order
to explain the accelerated motion of Encke's comet. But is this
the only case of acceleration? No. It appears from the re-
searches of M. Axel M oiler, a Swedish astronomer, that
Faye's comet presents a similar phenomenon; this follows
from an examination of the three successive revolutions it
has accomplished since the date of its discovery in 1861. 'The
motion of the comet,' he remarks, ' cannot be accounted for
by attraction only.'
Why, then, is it that these two comets alone have mani-
fested the operation of a cause whose action, whether pro-
duced by a resisting medium or a repulsive force, must be
general ? On Encke's hypothesis the short-period comet, being
that which, of all known periodic comets, or rather of comets
that have returned, approaches the nearest to the sun, would
necessarily experience most strongly the influence of a medium
which is denser in the regions nearer the sun. On the hypo-
thesis of a repulsive force the explanation is the same, since the
intensity of that force being greater at less distances from the
sun, its tangential component is likewise greater. But this
reasoning no longer holds good when we turn from the corsi-
deration of Encke's comet to that of Faye, whose perihelion
distance, and even mean distance, are, on the contrary, among
the greatest. Perhaps these apparent divergencies are due to
nothing more than a want of accuracy in the calculated move-
ments of these bodies and their perturbations. In any case
much remains to be done before we shall be enabled to decide
between the theories that have been, proposed.
413
CHAPTER XII.
COMETS AND SHOOTING STABS.
SECTION I.
WHAT IS A COMET ?
The ancients were unacquainted with the physical nature of comets — False ideas enter-
tained by astronomers of the eighteenth century respecting the physical constitution
of comets ; comets regarded by them as globes, nearly similar to the planetary
spheroids — Views of Laplace upon comets, compared by him to nebulae — Con-
temporary astronomers have confirmed these views and rectified the errors of the
ancient hypotheses — Desideratum of science ; the rencontre of the earth with
a comet or the fragment of a comet.
THE question, What is a comet ? examined in the preceding
chapter, and which we reproduce as the heading of this
Section, has been the subject of numerous hypotheses. It
cannot, however, yet be considered as answered. But it has
lately been attempted in an entirely new manner, and by a
method least of all to be expected — that of direct investigation.
The exposition of this method, and the considerations which
have led to it, will be the object of this new chapter.
Let us commence by recapitulating the substance of what
our previous enquiries and researches have already taught us.
The ancients, as we have seen at the commencement and
in the course of this work, held notions concerning the nature
of comets that were entirely hypothetical, and moreover con-
tradictory. On passing their conjectures in review it is
surprising, no doubt, to meet with ideas, to some extent, in
conformity with the accepted facts of modern science. But
417 E E
THE WORLD OF COMETS.
the astronomers of the Middle Ages and of the Renaissance,
up to the time of Newton, and even later, were not more
advanced than the ancients: the coincidences of which we
speak, therefore, are purely accidental. If we fabricate hypo-
theses entirely conjectural, we may occasionally chance upon
propositions so much in unison with the truth that they might
be for a moment regarded as the result of a marvellous divi-
nation ; but it i's not so. Xenophanes, for example, has called
comets wandering clouds, which is doubtless true; but what a
difference between the meaning attached by the philosopher
to such an expression and that given to it by science at the
present day !
We have already said that the progress of cometary
astronomy from the time of Newton, a progress essentially
mathematical or mechanical, caused the philosophers of the
eighteenth century to entertain very erroneous views con-
cerning the physical constitution of comets. Seeing only in
these bodies planets making longer voyages than others, and
having orbits more inclined to the ecliptic, they regarded them
almost as planets of the solar system. For these philosophers
they were globes surrounded with denser atmospheres, which
were subjected to extreme vicissitudes of heat and light, and
consequently differed greatly from those of the planets; in fact,
their climates were different, but that was all.
Laplace, guided by his own more profound views of the
origin of the solar and planetary world — views nevertheless pro-
pounded with reserve — was the first to see clearly that there
must be a difference of origin between comets and the planets,
as both the smallness of the masses and the optical appearance
of the former denote an essential difference of structure. Not
only does he look upon cornets as nebula?, but as wandering
nebulae — visitors for a time to our planetary world, wandering
from star to star or from system to system. Some few only,
418
WHAT IS A COMET ?
conquered for a time by the effect of the planetary pertur-
bations, become temporary satellites of the sun.
But did Laplace himself, whose views seem to gain credit
in proportion as science advances, attach to the word nebula
a well-defined physical signification? Evidently he neither
could nor would assimilate a comet to a resolvable nebula ;
that is to say, to a mass composed of a multitude of little stars.
In his opinion it was a confused mass of elements analogous to
the proper nebula of Herschel, in which that celebrated astro-
nomer saw matter being condensed to form centres of light, or
suns ; or rather it was, as it were, a portion of the primitive
solar nebula or of some other similar agglomeration.
What have science and observation added to these naturally
somewhat vague notions? This we have seen in the chapters
which treat in detail of the physical and chemical constitution
of comets, and from which it follows that a comet is altogether
differently constituted to the globes more or less similar to
the earth which form the planets. It is a mass in a state of
unstable equilibrium, whose form is modified with extreme
rapidity, according as it receives the solar radiations at a greater
or less distance, and which is subjected to the attraction of the
sun and of the planets. The transformations, physical, calorific,
luminous, and perhaps chemical, of the nebulous portion are so
extraordinary and rapid, that nothing in other bodies can furnish
an adequate idea of them. Everything leads us to believe that
cometary nuclei, even when they may be regarded as solid
masses, or composed of multiple solid masses, more or less
aggregated, are themselves the seat of transformations quite
as singular.
Spectral analysis, in spite of the few results it has as yet
furnished concerning the light of comets, gives us, nevertheless,
reason to suspect that the matter of cometary nebulosities is
chemically composed of but two or three elements at most:
419 E B 2
THE WOKLD OF COMETS.
while the matter of the nucleus, whether it be an incandescent
liquid or solid, or only solid matter reflecting the light of the
sun, quite eludes our research, nor do we know anything of the
chemical elements of which it is composed.
To improve our knowledge on these doubtful points of
cometary astronomy it would be necessary that one of those
events so much dreaded by the timid and superstitious should
come to pass. It would be necessary that our globe should
encounter a comet on its path, or let us say, as a more inoffen-
sive hypothesis, the fragment of a comet. Would not the
penetration of the cometary matter, into the atmosphere, its fall
upon the earth, by permitting men of science to contemplate
de visu and touch with their hands the substance of a comet,
put an end to all uncertainty?
Up to the present time there has been nothing, in the history
of mankind, nothing in the past history of the earth, geologically
considered, to indicate that such an event has ever taken place.
But even were it otherwise, and had such an event actually
occurred, supposing it to have been attended with no disastrous
consequences, science, which then did not exist, was in no
condition to profit by the occurrence, and we should be no
better informed than we are at present respecting the true
constitution of a comet.
Is it true that in 1861, on June 30, the earth passed through
the tail of the great comet which appeared at that time ? We
shall see in a subsequent Section that it is possible that such an
event took place. Several astronomers who have carefully studied
the question affirm that it did. And what was the result ? At
the utmost a phosphorescent glimmer ; certainly nothing was
either seen or felt. From what we know of cometary tails, their
mass and excessively small density, nothing could have been seen.
We should have to penetrate to the midst of the cometary
nucleus, or at least to the matter of the vaporous head, the
420
WHAT IS A COMET ?
luminous aigrette* and envelopes, in order to witness the
singular phenomena whose development the telescope has
revealed to us already. But the probabilities of such a ren-
contre are so slight that there is little chance that we shall ever
add to our knowledge by this means. The ether is so vast an
ocean, its abysses are so profound, so prodigious in extent com-
pared with the insignificant volumes of the stars, of our pigmy
earth and even of the larger comets, that the chance of a colli-
sion between our earth and any of these bodies, whose paths
are so well regulated, is very small.
But have we not other reasons for hoping for a rencontre,
if not between the earth and a comet, at any rate between the
earth and fragments of cometary matter? This would supply
the desideratum of science. We have seen comets divide into
two ; we have seen their nuclei project, under the influence of
unknown forces, waves and jets of matter to enormous distances
in space. Does the matter thus distributed finally regain
the centre from which it was projected ; or does it, on the
contrary, abandon it for ever, so that comets, in each of
their revolutions, lose a portion of their substance? This is
the question which we shall now examine.
421
SECTION II.
IS THE MATTER OF COMETS DISPERSED IN THE INTER-
PLANETARY SPACES?
AT a distance from the sun the nebulous agglomerations
which constitute a comet preserve a spherical or globular
form, a certain indication that their molecules obey the pre-
ponderating action of the nucleus. This form would be
preserved if no foreign influence interfered to derange their
mutual positions or to disturb the general equilibrium.
But the comet, when approaching its perihelion, is subjected
more and more to the attractive power of the sun, whose enor-
mous mass suffices to change the spherical form of the cometary
nebula, to render it more and more ellipsoidal, and finally to
carry away beyond the sphere of the attraction of the nucleus
whole strata of the nebulosity. This is proved beyond a doubt,
as we have seen, by the analysis of M. Roche. In addition to
the action of the solar mass there is likewise the action of
radiated heat from the sun, which determines changes of great
importance : the emission of vaporous matter from the nucleus,
luminous jets, aigrettes, and successive concentric envelopes.
If the tails of comets, as everything leads us to believe, are
material realities, and not simple visual effects; if they are
molecules detached from the nebulosity and projected far
beyond it by a repulsive force, we may say that, having passed
beyond the preponderating action of the nucleus, they have for
422
MATTER OF COMETS IN INTER-PLANETARY SPACES.
the moment become foreign to the comet itself, which lias thus
suffered a portion, however small, of its matter or its mass to
escape.
We have said for the moment. In fact, the molecules,
although no longer retained by the cometary nucleus, but
abandoned and repelled, have not for that reason lost the orbital
movement, or movement of translation, which they possessed
in common with the others. They continue, therefore, to
describe around the sun orbits which carry the whole of them
together through the same regions of space as the comet itself.
No sooner have they passed the points which form their respec-
tive perihelia than, like the comet, they recede more and more
from the sun, the focus of their movements. Now, in propor-
tion as the distance from the sun increases, the causes which
have led to their dispersion diminish in intensity, and the nucleal
attraction gradually resumes the influence it originally pos-
sessed. We do net see, then, why the whole of the dispersed
molecules should not ultimately return, if not to resume their
original places in the system, at least to reconstitute the nebu-
lous agglomeration, not in the same form, no doubt, but so as
to reproduce the primitive mass.
In point of fact, however, for matters to happen exactly in
this way we should have to suppose that no cause of perturba-
tion could arise to derange the system or disturb the re-con-
stitution. Now, we know that comets traverse the planetary
system, and project to enormous distances across it the matter of
their tails. The nuclei themselves are disturbed in their orbits
by the masses of Jupiter, Mars, and the Earth. This same dis-
turbing action must exert its influence upon the detached
nebulosities of comets and carry away for ever from the latter
the outlying portions of their atmospheres and tails. Such an
intervention doubtless divided in two the comet of Biela, and
has since dispersed one or other of the fragments. What, in
423
THE WORLD OF COMETS.
fact has become of these debris which failed to reappear at the
time when their regular period should have brought them
within sight of our globe ?
Thus everything inclines us to believe that scattered here
and there upon the planetary shores, or floating upon the waves
of the ethereal ocean, shattered and broken-up comets exist;
remains of shipwrecks suffered by millions of comets ; waifs of
aerial barks unable to accomplish the voyage without paying
tribute to the element in which they float. Nevertheless such
fragments, more or less disintegrated, do not wander at will in
space ; they move in orbits whose forms are dependent upon
the modifications which the disturbing influence has brought to
bear upon the original motions. We may suppose that these
orbits continue to be closed curves of which the sun is the focus,
or that they have become changed to hyperbolas. We may
even suppose the disturbing influence to have been such that
these disintegrated fragments have become satellites of the
disturbing planet. Perhaps all these results and many more
have been brought about in the course of time.
The number of comets which penetrate into our world is,
in all probability, so immensely great, that during the hundreds
of millions of years which we may assume to have elapsed since
the beginning of the world the interplanetary spaces must have
been furrowed by prodigious multitudes of currents of matter
from disintegrated comets, fragments of comets, which the
planets, in their regular course about the sun, cannot fail fre-
quently to encounter. It can hardly be otherwise. And indeed
at the present day there is nearly certain evidence that this
actually is the case ; and, although originally the proof of the
existence of these material currents may have been arrived at
in another manner, the fact of their existence is certain. Their
origin alone can be matter of doubt.
424
SECTION III.
COMETS AND SWARMS OF SHOOTING STARS.
Periodicity of the meteor-swarms ; radiant points ; number of swarms recognised at
the present day — Periodical maxima and minima in certain meteoric currents;
thirty years period of the November swarm — Parabolic velocity of shooting stars ;
the swarms of shooting stars come from the sidereal depths.
THESE considerations bring us to the theory recently elaborated
by the learned Italian astronomer M. G. Y. Schiaparelli, Director
of the Observatory of Brera, at Milan.
According to this theory there exists between comets and
shooting stars a connexion and community of origin, which
henceforth we may regard as certain, as it is supported both
by logical deduction and observation. We shall now explain
by what train of ideas this assimilation between phenomena
which at first sight appear so foreign to each other has passed
from the phase of simple hypothesis into that of a theory which
observations of great value permit us at the present time to
consider demonstrated.
Let us first of all pass in review the facts upon which the
theory is based.
The shooting stars which may be observed on any clear
night throughout the year are notably more numerous at cer-
tain times, the dates of which are nearly fixed, as, for example,
August 10, November 13 or 14, and April 20. They then
appear in sufficient numbers to be considered, not as isolated
425
THE WORLD OF COMETS.
meteors, but as groups or swarms of meteors. This connexion
soon became still more manifest when it was found that the
stars of each swarm were moving in trajectories, not distributed
at random over the celestial vault, but so disposed that if
prolonged backwards all or nearly all passed, if not through
the same mathematical point, at least through one very circum-
Fig. 69. — Shooting Stars of November 13-14, 1866. Convergence of the tracks,
according to A. S. Ilerschel ami A. MacGregor.
scribed region of the heavens. To obtain an idea of this
remarkable conjunction of the apparent trajectories of a stream
of shooting stars the reader has only to glance at fig. 69,
representing the tracks among the stars of a certain number
of these meteors which appeared on the night of Novem-
ber 13-14, 1866.
426
COMETS AND SWARMS OF SHOOTING STARS.
The point of emanation or convergence of the trajectories
of a swarm is called the radiant point. Now, not only have
the meteors which appear at the fixed dates we have mentioned,
both on the same night and on several successive nights, the
same radiant point, but this radiant point does not vary in
position, or varies very little, in the course of years for succes-
sive apparitions of the same meteor-current.
At the time when researches were first commenced re-
specting these singular phenomena the known streams of
periodical return were few in number ; those of August 10
and November 14 were at first alone recognised. It was
thought that the shooting stars of ordinary nights were
scattered without any apparent connexion. But more careful
observations showed that in reality these sporadic shooting
stars obeyed la\vs similar to those of the other swarms ; a
great number of streams were thus distinguished, and the
positions of their radiant points determined. At the present
time 102 are known.*
But another fact of great importance was established, partly
by historical researches in regard to the previous apparitions of
similar phenomena, and partly by the continuous observations
of contemporary astronomers. The fact in question is this : —
The periodicity of meteoric swarms is not only annual, so
that the same nights of each succeeding year are remarkable for
displays of meteors sufficiently abundant to distinguish these
nights from those immediately adjacent, but it also happens
that, as regards certain streams, the display varies in a manner
which clearly enables us to distinguish the recurrence of peri-
odical maxima and minima. We will cite a remarkable in-
stance of this periodicity, viz. that of the swarm of the middle
of November. In 1799 the stream of shooting stars was of
* [See addition to this chapter. — ED.]
427
THE WORLD OF COMETS.
wonderful intensity. The same phenomenon was repeated
thirty-four years later ; that is to say, in the year 1833. On
tracing back the records of similar phenomena the display of
1766 was found to have been not less abundant at the same
date. Similar instances were likewise found at more remote
epochs, and it appeared that the intervals between these ex-
traordinary apparitions were always either thirty-three or thirty-
four years, or a multiple of these numbers. It seems certain,
then, that for this particular swarm there is a periodical maxi-
mum which recurs about every third of a century. Thus Olbers
did not hesitate to predict, long beforehand, the recurrence of
a maximum for the year 1867. Professor Newton, of America,
furnished more precise data, and denned the period with greater
accuracy, assigning to it a duration of thirty-three years and a
quarter, and fixing the date of its next return for the night of
November 13-14, 1866.
The meteor-swarm was true to the prediction.
It soon became clear that these singular phenomena could
only be explained by considering the different swarms of
meteors as of extra-terrestrial or cosmical origin. Other cir-
cumstances, which we shall enlarge upon in a separate work,
contributed, in conjunction with those which we have just
related, to prove that shooting stars form currents of celestial
particles which circulate independently in space and describe
regular orbits like comets. These numerous currents traverse
the interplanetary spaces in all directions, and by their occa-
sional rencontres with the earth give rise to the production of
shooting stars. Our globe, or simply its atmosphere, pene-
trating more or less deeply to the centre of one of these
groups, a meeting takes place, the velocity being sometimes
equal to the sum and sometimes to the difference, or to any
intermediate amount, of the respective velocities of the two
bodies. In any case there is a loss of vis viva, or rather a trans-
428
COMETS AND SWARMS OF SHOOTING STARS.
icrcnce of vis viva into heat, which generally produces incan-
descence.
But, if these swarms are currents of meteoric matter, what
law governs their circulation in space ? What are the elements
of their orbits ? And last, and most interesting of all, what is
their origin? These questions called forth various hypotheses
in reply. The swarms, it was supposed, were closed rings
more or less elliptic and more or less eccentric in regard to the
sun, the focus of their movements. In this way the facts of
observation were accounted for.
In this state of the question, M. Schiaparelli, by some bold
speculations, succeeded in throwing a new light upon an
obscure point in the theory. He believed it could be shown
from different observations that the velocity of meteors at the
moment of their entrance into the terrestrial atmosphere was
at least equal to what is termed cometary velocity, and conse-
quently nearly half as much again as the velocity of the earth's
translation. He showed that this hypothesis accounted for a
fact* which at first appeared to be inconsistent Avith the
cosmical origin of shooting stars, but which was, on the con-
trary, a striking confirmation of it. Provided with this im-
portant element, the Italian astronomer was enabled to calculate
the elements of the orbits of certain swarms, and he found
that they describe in space excessively elongated curves,
parabolas or hyperbolas.
* That the number of shooting stars observed on any night varies with the
time of observation. The maximum horary number takes place in the hours 2
or 3 A.M. [for the November meteors.]
429
SECTION IV.
COMMON ORIGIN OF SHOOTING STARS AND COMETS.
Transformation of a nebula which has entered into the sphere of the sun's attraction ;
continuous parabolic rings of nebulous matter — Similarity between the elements of
the orbits of meteor streams and cometary orbits — The August stream ; identity
of the Leonides and the comet of 1862— Identity of the Perseids and the comet of
1866 (Tempel)— The shooting stars of April 20 and the comet of 1861 — Biela'a
comet and the December stream — Did the earth encounter Biela's comet on No-
vember 27, 1872 ?
IT still remains to explain the origin of meteor swarms or
streams, and the reason of their annual periodicity and the
maxima which appear at dates separated by intervals of several
years. For this purpose it will be necessary for a moment to
quit the domain of fact and consider some theoretical specu-
lations.
The swarms of shooting stars appear to be constituted, as
it were, of aggregations of particles separated from one another
by some distance. But if, instead of seeing them on their
arrival in the proximity of the earth, in contact with its atmo-
sphere, it were possible to contemplate them from a distance
in the heavens, the whole of these myriads of particles,
whether illuminated by the sun's rays or shining by their own
light, would appear to the observer like a cloud or nebulosity.
And as the supposed velocity of meteoric swarms in their
orbits is that of cometary velocity, it follows that the nebu-
losities of which we speak come from the depths of space,
430
COMMON ORIGIN OF SHOOTING STAIIS AND COMETS.
from regions far beyond the sun and the planets. Never,
theless, it is certain that these nebulte, which have, perhaps,
quitted some other sidereal system, only make their entrance
into ours under the influence of a momentary preponderance
of the attraction of our sun.
It was, doubtless, from considerations of this kind that
Schiaparelli was led to propound the following problem : Given
a nebulosity situated at a very great distance, but never-
theless such that the attraction of the sun determines its
movement towards our system, what will be the form of this
aggregation (supposed to be spherical at starting) when it
arrives at its perihelion ? Resolving this problem by analysis,
and according to the principle of gravitation, M. Schiaparelli
proves that the nebulous mass, though globular at first, will
be gradually transformed, so as to be, at the time of its
passage in the vicinity of the sun, converted into a continuous
stream or current of parabolic form, very much more dense
than it was originally, and taking hundreds and even thou-
sands of years to effect completely its perihelion passage.
Hence it will be understood that the earth encountering
o
this stream at one point of its orbit, and passing, after each of
its revolutions, through this same point of interplanetary
space, a periodic apparition of meteors will take place; par-
ticles of the stream traversing the higher regions of the atmos-
phere will shine each for a moment as a shooting star, some to
be totally destroyed by their combustion, others to pursue their
course, after having thus manifested for a moment the presence
of the nebula of which they form a part. These long para-
.bolic trains explain the yearly-recurring streams of meteors: ac-
cording to the greater or less thickness of the portion of the
stream traversed by the earth will the number of shooting
stars seen be more or less considerable.
As regards the longer periods which give the maxima at
431
THE WORLD OF COMETS.
regular intervals of several years, M. Schiaparelli accounts for
them as follows : — ' In the same manner as the long parabolic
currents are comparable, as far as their movements are con-
cerned, to comets of infinite orbits, intermittent periodic
streams are analogous to periodic comets of regular return.
Incidental circumstances — the planetary perturbations, for ex-
ample— may transform an endless stream into an elliptic closed
ring.' The meteors of November 13 and 14 are, very pro-
bably, according to Schiaparelli, a case in point.
VjrvV
\ \ A .--""ANi i \V
Fig. 70. — Orbits of the Meteoric Streams of November, August, and April, and of the
Comets of 1866 and 1861.
We shall not enter more particularly into the details of
this remarkable theory, but shall confine ourselves to pointing
out the analogy existing between the nebulous currents which
give birth to the meteoric swarms and the nebulosities of
comets. The velocity of translation, the inclinations of the
planes of the orbits at all angles, the movements in all direc-
432
COMMON ORIGIN OF SHOOTING STARS AND COMETS.
tions, are elements common alike to the comets and to the
meteor swarms.
One most essential sanction was still wanting to this
theory, that given by observation, alone capable of actually
demonstrating the connexion between the two kinds of nebula;.
Now at the present time this sanction exists ; it soon followed
the theory, and appears evident enough to defy all contra-
diction. M. Schiaparelli, having collected the observed elements
of the meteoric swarm of August 10, was enabled to calculate
its orbit as if it had been that of a celestial body or a comet.
On examining the parabolic elements of comets which had
been catalogued he recognised the almost identity of the ele-
ments of one of these comets with those of the meteor
swarm. The following table of elements will exhibit this com-
parison : —
Elements of the orbit of
the meteoric stream of
August 10, calculated
by Schiaparelli
Elements of the orbit of
the comet of 1862, cal-
culated by Oppplzer
Perihelion passage
August lO'lo.
August 22-9, 1862.
Longitude of perihelion
343° 28'
344° 41'
Longitude of node
138° 16'
137° 27'
Inclination .
64° 8'
66° 25'
Perihelion distance
0-9643
0-9626
Direction of motion
Retrograde.
Retrograde.
A similarity so complete could hardly be an accidental co-
incidence. But M. Schiaparelli did not confine himself to this
one comparison. He calculated in the same manner the ele-
ments of the orbit of the November stream, and found that
they were almost identical with the elliptic elements of
Tempers cornet (1866 I.), calculated by Oppolzer. The
following table affords the means of comparing these ele-
ments : —
433 , F F
TTIE WOULD OF COMETS.
Elliptic elements of tlic
orbit of the meteoric
stream of November 13,
calculated by Schia-
Elliptic elements of the
orbit of Tempel's comet
(1866, I.), calculated
by Oppolzer
parelli
Perihelion passage
November 10-092
January 11-160, 1866
Longitude of perihelion
50° 25' 9"
60° 28' 0"
Longitude of node
231° 28' 2"
231° 26' 1"
Inclination .
17° 44' 5"
17° 18' 1"
Perihelion distance
0-9873
0-9765
Eccentricity
0-9046
0 9054
Semi-axis major .
10-340
10-324
Period
33-250 years.
33'176 years.
Direction of motion
Retrograde.
Retrograde.
After having pointed out this new and important coinci-
dence, M. Schiaparelli observes: ' It is very worthy of remark
that the two well-known meteor streams, those of August and
November, have each their comet. Are we to suppose that it
is the same with all the others ? If so, we should be forced to
regard these cosmical streams as the result of the dissolution
o
of cometary bodies. But it would be at least premature to
extend this conclusion to all shooting stars. It is possible, as
I have shown, that the whole of these bodies, great and small,
may form systems in space bound together solely by their own
attraction, and afterwards destroyed by the action of the sun.
Perhaps, also, that which we term a comet is not a single
body, but a collection of very numerous and minute bodies,
attached to a principal nucleus.'
We cannot fail to notice the connexion existing between these
views and those which led M. Hoek to his study of the theory
of cometary systems ; we must also perceive how completely
these new views on the subject of cometary physics accord
with the facts of observation that have been mentioned in a
preceding section respecting the duplication of Biela's comet
and the division and shattering of ancient comets, phenomena
which have been handed down to us by tradition, but which
434
COMMON ORIGIN OF SHOOTING STARS AND COMETS.
had hitherto been generally denied and regarded as fables by
astronomers.
In conclusion, and before leaving so vast a domain, open
alike to new researches and conjectures, we must not forget to
mention two more cases of identity between meteoric swarms
and comets. The first relates to the meteors of April
20. According to MM. Galle and Weiss the orbit of this
swarm has the same elements as the orbit of the comet of
1861. D'Arrest and Weiss have likewise found an accordance
between Biela's comet and the shooting stars of the end of No-
vember and the first days of December. We have already said
that the remarkable shower of shooting stars which distin-
guished the night of November 27, 1872, appears certainly to
have been due to the rencontre of the earth either with one
of the two comets, fragments of that of Biela, or with a stream
of matter which originally belonged to that comet, and which
followed in space nearly the same course.*
If these views — which would have appeared so strange half
a century ago — should be confirmed, we have a new and quite
unexpected means of putting ourselves in direct communication
with comets, since the earth every year — every night in the
year, indeed — comes in contact with nebulosities which have
been comets. A new light would be thrown upon the phy-
sical constitution of these bodies, and we might then consider
as highly probable that granulated structure of cometary nu-
clei, formed of isolated particles, which Babinet was led to
suspect upon very different grounds.
* [See note, p. 2G5.— ED.]
43-") F P 2
ON THE CONNEXION BETWEEN COMETS AND
METEORS.
BY THE EDITOR.
The intimate connexion now known to exist between
comets and meteors is perhaps the most striking and novel dis-
covery of a purely astronomical kind that has been made in
our time. To those who are aware how few years have elapsed
since the apparitions and tracks of meteors seemed to be so
arbitrary and capricious that in the opinion of many it was
scarcely worth while to record them, it cannot but be matter
of wonder to consider how great has been the advance in our
knowledge, and how rapid has been the progress of ideas on
this subject. On account of the importance of the results
found upon the nature of comets, I, therefore, add here several
details, chiefly historical, which will serve to show more fully
how remarkable is the connexion that has been established.
It is less than ten years since the orbit of the first stream
of shooting stars — that of the middle of November — was calcu-
lated, the circumstances being as follows : —
Professor H. A. Newton, of the United States, collected
and discussed thirteen historic showers of the November
meteors between the years 902 and 1833.
The following table exhibits these displays, and the earth's
longitude at each date, together with the same particulars for
the shower of 1866 : —
436
COMMON ORIGIN OF SHOOTING STARS AND COMETS.
A.D.
Day and Hour.
Earth's longitude.
902
October 12-17
24° 17'
931
, 14-10
25° 57'
934
, 13-17
25° 32'
1002
, 14-10
26° 45'
1101
, 16-17
30° 2'
1202
, 18-14
32° 25'
1366
, 22-17
37° 48'
1533
24-14
41° 12'
1602
, 27-10 (Old style)
44° 19'
1698
November 8' 17 (New style)
47° 21'
1799
, 11-21
50° 2'
1832
, 12-16
50° 49'
1833
12-22
50° 49'
1866
13-13
51° 28'
From these data Professor Newton inferred that these dis-
plays recur in cycles of 33^ years, and that during a period of
two or three years at the end of each cycle a meteoric shower
may be expected. He also concluded that the November
meteors belong to a system of small bodies describing an elliptic
orbit about the sun, and extending in the form of a stream
along an arc of that orbit which is of such a length that the
whole stream occupies about one-tenth or one-fifteenth of the
periodic time in passing any particular point. He further
showed that the periodic time must be 180 days, 185 days,
355 days, 377 days, or 33£ years, and suggested that by calcu-
lating the secular motion of the node for each one of the five
possible orbits, and by comparing the values with the observed
motion (about 52" annually or 29' in 33| years) it would be pos-
sible to decide which of these five orbits was the correct one.
Soon after the remarkable display of the November meteors
in 1866, Professor Adams, of Cambridge, undertook the ex-
amination of this question. Beginning with the orbit of 355
days, which Professor Newton considered to be the most pro-
bable one, Professor Adams found the motion of the node
would only amount to 12' in 33£. years ; that for the orbit of
437
THE WORLD OF COMETS.
377- days the value would be nearly the same, while if the
periodic time were a little greater or a little less than half a
year, the motion of the node would be still smaller. It there-
fore only remained to examine the orbit of 33 1 years, and Pro-
fessor Adams found that, for this orbit, the longitude of the
node would be increased 20' by the action of Jupiter, nearly 1'
by the action of Saturn, and about 1' by the action of Uranus.
The other planets produce scarcely any sensible effects, so that
the entire calculated increase of the longitude of the node in
the above-mentioned period is about 29'. As already stated, the
observed increase of longitude in the same time is 29', and this
remarkable accordance between the results of theory and obser-
vation left no doubt as to the correctness of the period of 33^
years.*
Subsequently, however, to the commencement, but before
the publication of Professor Adams's results, M. Schiaparelli
had been led, on totally different grounds, to conclusions which
first suggested a probable connexion between meteors and
comets. These related to the August meteors, or Perseids as
they are called from the constellation which usually contains
their radiant point. A comparison of the average hourly in-
crease in the frequency of meteors, throughout the year, from
evening until daybreak, with a mathematical formula for the
same variation in terms of their velocity, led M. Schiaparelli
to conclude that the real average velocity of shooting-stars in
their orbits round the sun did not differ much from that of
comets moving in parabolic orbits, which is greater than the
earth's mean orbital velocity at the same distance from the sun
in the proportion of 1-414 to 1. Discussing, then, the origin
of meteoric currents M. Schiaparelli remarked that in all re-
spects shower-meteors resembled comets rather than planetary
* Monti,/!/ Notices of the Roy. Ant. Soc., vol. xxvii., p. 250 (April 18G7).
438
COMMON ORIGIN OF SHOOTING STAltS AND COMETS.
bodies indigenous to the solar system ; and shortly afterwards
supposing groups of meteors to have entered our system ori-
ginally as cosmical clouds, he formulated his theory in a series
of nine propositions, of which I extract two. ' IV. Whatever
may be the shape and size of a cosmical cloud, it can rarely
enter the central parts of the solar system without being trans-
formed into a parabolic current, which may occupy years, cen-
turies, or thousands of years in completing its perihelion pas-
sage, in the form of a stream extremely narrow in comparison
with its length. Of such streams those which the earth en-
counters in its annual revolution present themselves as a shower
of shooting-stars diverging from a common centre of radiation.'
' VIII. Shooting-stars and other like celestial bodies, which in
the last century were regarded as atmospheric, which Olbers
and Laplace first maintained might be projected from the moon,
and which afterwards came to be regarded as planetary bodies,
are in reality bodies of the same class as the fixed stars ; and
the name of falling stars, applied to them, simply expresses the
real truth. They bear the same relation to comets which the
planetoids between Mars and Jupiter bear to the larger planets,
the smallness of the masses, in both cases, being compensated
for by the greatness of their number.' *
Subsequently M. Schiaparelli showed that if the Perseids
be supposed to describe a parabola, or a very elongated ellipse,
the elements of their orbit calculated from the observed posi-
tions of their radiant point agree closely with those of the orbit
of Comet II., 1862, as calculated by Dr. Oppolzer ; so that it
seemed probable that the great Comet of 18G2 was part of the
same current of matter as that to which the August meteors
belong : he also gave approximate elements of the orbit of the
November meteors (or Leonids), calculated upon the supposition
* Bullettino Meteorologico dell' Ossetratcrio del Collegia Romano, vol. v.
THE WORLD OF COMETS.
that the period of the revolution was 33 J years : but as
the calculations were founded on an imperfect determination of
the radiant point, he failed to find any cometary orbit that
could be identified with that of the meteors. A few weeks
later M. Le Verrier gave more accurate values of the elements
of the November meteors, his calculations being based upon a
better determination of the radiant point, and M. Peters, of
Altona, pointed out that these elements closely agreed with
those of Tempel' s comet (I., 1866) ; an agreement which M.
Schiaparelli, who had recalculated the elements of the orbit of
the November meteors, also remarked independently.
Thus we see at very nearly the same time M. Schiaparelli
was enabled to identify the orbit of the August and November
meteors with those of two known comets, and Professor Adams
placed almost beyond doubt the fact that the November meteors
did actually describe an orbit round the sun in 33^ years ; so
that almost simultaneously it was shown that the November
meteors did describe their orbit in 33J years ; and that assum-
ing this, the orbit resembled very closely that of Tempel's
comet of 1866.
The reader will readily see from fig. 69 (p. 426), that the
tracks of the meteors, as laid down upon a celestial chart,
intersect in a very restricted region of the sky, but that this
region is very far from being an exact point. The same will
be seen in tig. .4, which shows the tracks of certain of the
meteors observed at Greenwich, on the nig-ht of the great
' O O
star-shower of November 1866. This want of precision is
partly due to the extreme difficulty of noting down quite
accurately the track of a meteor from a rapidly made eye-
observation, and partly to the fact that the meteors do not
all proceed from the same mathematical point. Thus the
determination of the radiant point, and therefore of the orbit,
of the meteors is always a matter subject to some slight
440
COMMON ORIGIN OF SHOOTING STARS AND COMETS.
amount of uncertainty. It will be remarked that in fig. A,
the tracks are not prolonged backwards as in fig. 69, so that
.the apparent lengths of the meteor-tracks are shown, and
that these are shorter in proportion as they are nearer to
the radiant point. This is merely an effect of perspective,
for a meteor seen at the radiant point would be directly
approaching us, and therefore would merely appear like a
FIG. A. — Tracks of Meteors observed at the Royal Observatory, Greenwich, on the night of
November 13—14, 1866.
motionless star, destitute of train, appearing and then disap-
pearing; and the tracks would be longer the further removed
they were from the radiant point.
In fig. B, is shown the rate of frequency of the meteors
in the same remarkable star-shower. Eight of my observers,
at the Royal Observatory, Greenwich, were engaged in count-
ing the number of shooting- stars per minute, each taking a
441
THE WOULD OF COMETS.
different portion of the heavens. The maximum number was
therefore attained between 1 and 2 A.M. on the morning of
the 14th, and the display then rapidly subsided. The curve,
in fig B is interesting as it may be regarded as representing
Nujnber
per
Minute
120
Hours of Observation
jo PM 11 Mult
1AM
nsvirfo
Fiu. B. — Showing the rate of frequency -of Meteors seen per minute at the Royal
Observatory, Greenwich, on the night of November 13 — 14, 1866.
the density of the meteors in the stream that we were
crossing.
The August meteors (the Perseids) are much less nume-
rous than those of November, but they appear every year,
442
COMMON ORIGIN OF SHOOTING STARS AND COMETS.
and large meteors are frequent among them. Fig. C gives a
number of tracks of these meteors observed by Professor
Tachini in 1868.
It was this meteor-swarm that M. Schiaparelli first iden-
tified with a comet (Comet II. 1802).
FIG. C. — Tracks uf Meteors recorded by Professor Tachini at Palermo on August 8
to August 12, 1868.
The number of known radiant points has in the last few
years — now that the subject of meteoric astronomy has begun
to excite attention — been greatly increased, and now exceeds
seven hundred/'"
* References to 695 radiant points will be found in Monthly Notices of the
Royal Ast. Soc., vol. xxxv., p. 249 (1875).
THE WORLD OF COMETS.
The fact that there is a close connexion between comets
and meteors is all that can be considered established with
certainty ; but M. Schiaparelli has developed his views on
their relation to one another at some length. According to
him, comets and meteoric groups are portions of the nebulous
matter in space in two different states of condensation, which
may arise either together or apart, according to the tenuity
of the matter which produced them. Such differences are
observable in the nebula? of which some are resolvable and
others not resolvable in the telescope. Comets with two or
FIG. D. — (1) Nucleus of the Comet of 1618, observed telescopically by Cysatus.
(2) Comet of 1652, as seen December 27, according to Hevelius.
more nuclei have several times been observed. Figs. 61 and
62 (p. 341) exhibit the multiple nuclei of the comets of 1618
and 1661, according to Hevelius, and fig. D shows the nucleus
of the comet of 1618 according to Cysatus, and the comet
of 1652 which, according to Hevelius, consisted of a disk of
pale light of the apparent diameter of the full moon, arid in
the telescope appeared to be filled with points of light. M.
Schiaparelli considers that the various characters of all
these nebulous bodies can only be explained upon the hypo-
444
COMMON ORIGIN OF SHOOTING STARS AND COMETS.
thesis that they have been condensed from a state of highly
heated vapour, gradually undergoing a process of cooling and
condensing of its parts. ' Not only the various features of
star-showers and comets, but even the mineralogical struc-
ture of aerolites appear to be explained on this supposition.
The theory of Faye that they are developed from the nuclei,
and of Secchi that they are the remnants of the tails, and
of Erman that they are particles detached from comets by
a resisting medium, are not so immediately referable to the
known laws of gravitation as the hypothesis that all classes
of luminous meteors, like comets themselves, are drawn
from the sun by its attraction from the regions of intra-
stellar space, which the telescope declares to be empty, but
which, in all probability, are strewed with cosmical clouds,
containing in one order of phenomena both meteoroids and
clouds.' *
It would require too much space to enter further into
M. Schiaparelli's theory and explain how he accounts for
accidental or sporadic meteors, not apparently belonging to
any stream, &c. The reader will, however, see that in a
great many different respects the close relation of comets to
meteor-swarms seems to be established.
In a note at the end of Section iii., Chapter VII. (p. 265),
I have given an account of the remarkable circumstances
o
connected with the meteor-shower of November 27, 1872,
and Biela's comet ; and in relation to that note the following
particulars, due to Professor H. A. Newton, will not be found
uninteresting. The line of nodes, or the place of the earth's
nearest approach to the comet's track, being at N (fig. E), it
appears that in the year 1798, when the earth encountered
at that point the great meteor-shower of December 6 of that
* British Association Report, 1868, p. 414.
445
THE WORLD OF COMETS.
year, Biela's comet was in the position marked c, somewhat
nearer to the earth than on the next occasion, when a similar
occurrence was observed in 1838. The comet was in 1838
at the point A, about 300 millions of miles distant, measured,
along its orbit, from the earth. At the apparition of the
star- shower on November 27, 1872, the comet should have
occupied the position B, at about 200 millions of miles along
the comet's path from the place of the ea,rth's intersection
with the meteor- stream at N. It would thus appear that a
FIG. E.— Positions of Biela's Comet relatively to the Earth in 1798, 1838, and 1872.
long-extended group of meteor-particles must accompany the
comet in its periodical revolution, preceding it to a distance
of 300 millions of miles in front, and following it to a length
of 200 millions of miles in the rear; so that, as there is no
reason to suppose this elongated meteor-current discontinuous,
it occupies fully 500 millions of miles in its observed length
along the comet's path.*
* British Association Report, 1875, p. 224.
440
COMMON ORIGIN OF SHOOTING STARS AND COMETS.
But, as has been shown in the note already referred to,
there is considerable doubt with regard to the actual motion
of the comet, so that these conclusions must be regarded as
o
somewhat uncertain.
Professor Taifs Theory of the Constitution of Comets.
Professor P. G. Tait has recently published his theory of
the constitution of comets, which he assimilates to swarms
of stones or meteors, which are partly illuminated by the
sun, and also give out a light of their own through the
numerous and violent collisions which are always taking
place among them, especially near the nucleus, where the
swarm is densest. After stating that the researches of Mr.
Huggins have shown the presence of some hydrocarbon in
the comet, Professor Tait proceeds : —
' Now this is a most remarkable phenomenon. It at once
suggests the question — How does the hydro-carbon get into
this incandescent state in the head of a comet ? A word or
two on that subject may be of considerable interest, but we
must lead up to it gradually. A great astronomical discovery
of modern times is, that meteorites, the so-called falling stars,
especially those of August and November, as they are called,
follow a perfectly definite track in space, and that this track
is, in each case, the path of a known comet; so that — whether,
as Schiaparelli and others imagine, the meteorites are only a
sort of attendants on the comet ; or whether, as there is, I
think, more reason to believe, the mass of meteorites forms
the comet itself — there is no doubt whatever that there is
at least an intimate connexion between the two. The path
of the meteorites is the path of the comet. Well, let us con-
sider a swarm of such meteorites (regarded each as a frag-
447
THE WORLD OF COMETS.
ment of stone), like a shower in fact, of Macadamised stones,
or bricks, or even boulders. What would be the appearances
presented by such a cloud ? It must in all cases be of enor-
mous dimensions, because the earth takes two or three days
and nights to pass through even the breadth of the stratum
of the November meteors. Consider the rate at which the
earth moves in its orbit, and you can see through what an
enormous extent of space these masses are scattered. Now
if you think for a moment what would be the aspect of such
a shower of stones when illuminated by sunlight, you will
see at once that, seen from a distance, it would be like a
cloud of ordinary dust ; and an easy mathematical investi-
gation shows that it should give when sufficiently thick,
except in extreme cases, a brightness equal to about half
that of a solid slab of the same material similarly illuminated.
The spectrum of its reflected or scattered light should be the
spectrum of sunlight, only a great deal weaker. It is easy
without calculation, but by simply looking at a cloud of dust
on a chalky road in sunshine, to assure oneself of the pro-
perty just mentioned of such a cloud of dust or small par-
ticles. Remember that in cosmical questions we can speak
of masses like bricks or even paving-stones, as being mere
dust of the solar system, and we may suppose them as far
separated from one another, in proportion to their size, as
the particles of ordinary dust are. Whether, then, it be
common terrestrial dust, or cosmical dust, with particles of
the size of brickbats or boulders, does not matter to the
result of this calculation. Spread them about in a swarm
or cloud as sparsely as you please, and only make that
cloud deep enough, and illuminate it by the sun, then it can
send back one half as much light as if it had been one con-
tinuous slab of the material.
448
COMMON ORIGIN OF SHOOTING STARS AND COMETS.
1 Now, look at the moon. You see there a continuous
slab of material, and you know what a great amount of bright-
ness that gives. And a shower of stones in space at the same
distance from the sun as the moon, and of the same material as
the moon, could, if it were only deep enough, however scattered
its materials, shine with half the moon's brightness. Now no
comet's tail has ever been seen with brightness at all compar-
able to that of the moon ; and therefore it is perfectly possible
and, so far as our present means enable us to judge, it is
extremely probable, that the tail of the comet is merely a
shower of such stones. But now we come to the question,
How does the light from the head of the comet happen to con-
tain portions obviously due to glowing gas, in addition to
other portions giving apparently a faint continuous spectrum
of sunlight and perhaps also light from an incandescent solid ?
The answer is to be found — at least so it appears to me — in the
impacts of those various masses upon one another. Consider
what would be the effect if a couple of masses of stone or of
lumps of native iron such as occasionally fall on the earth's
surface from .cosmical space, impinged upon each other even
with ordinary terrestrial, not with planetary velocities. In
comparison with these latter, of course, the velocity of the shot
of any of the big guns at Shoeburyness would be a mere trifle ;
yet we know that when a shot from one of them impinges upon
an iron plate there is an enormous flash of light and heat,
and splinters fly off in all directions. Now mere differences
among the cosmical velocities of the particles of a comet, due
to different paths round the sun, or to mutual gravitation
among the constituents of a cloud, may easily amount to
1,400 feet per second, which is about the rate of a cannon-ball.
Masses so impinging upon one another will produce several
effects ; incandescence, melting, development of glowing gas,
449 G G
THE WORLD OF COMETS.
the crushing of both bodies, and smashing them up into frag-
ments or dust, with a great variety of velocities of the several
parts. Some parts of them may be set on moving very much
faster than before ; others may be thrown out of the race alto-
gether by having their motions suddenly checked, or may even
be driven backwards ; so that this mode of looking at the sub-
ject will enable us to account for the jets of light which sud-
denly rush out from the head of a comet (on the whole
forwards) and appear gradually to be blown backwards, whereas
in fact they nre checked partly by impacts upon other particles,
partly by the comet's attraction. Therefore so far as can be
said until we get a good comet to which to apply the spectro-
scope, this excessively simple hypothesis appears easily able to
account for many even of the most perplexing of the observed
phenomena. I must warn you, however, that this is not the
hypothesis generally received by astronomers.' (Lectures on
some recent Advances in Physical Science, pp. 254-257, 1876.)
Sir William Thomson in his Address before the British
Association in 1871, referring to Prof. Tait's theory — not then
published — explained that according to it, the comet, ' a group
of meteoric stones, is self-luminous in its nucleus, on account
of the collisions among its constituents, while its tail is merely
a portion of the less dense part of the train illuminated by sun-
light, and visible or invisible to us according to circumstances,
not only of density, degree of illumination, and nearness, but
also of tactic arrangement, as of a flock of birds or the edge of
a cloud of tobacco-smoke.'
It seems not at all improbable that this may be the real
explanation of the constitution of comets ; but it is clear that
only a complete mathematical investigation of the motion and
appearance of such an assemblage of particles, so illuminated,
can decide whether this theory will account for the; observed
450
COMMON ORIGIN OF SHOOTING STARS AND COMETS.
phenomena. It is clear that the mechanical difficulties which
the motion of a comet's tail presents on the hypothesis of its
consisting of matter moving with the comet have to be met ;
and this can only be effected by a thorough discussion of the
complicated dynamical problem involved.
451
CHAPTEE XIII.
COMETS AND THE EARTH.
SECTION I.
COMETS WHICH HAVE APPROACHED NEAREST TO THE EARTH.
The memoir of Lalande and the panic of the year 1773— Letter of Voltaire upon the
comet — Announcement iu the Gazette de France and the Memoirs of Bachaumont —
Catalogue given by Lalande of comets which up to that time had approached
nearest to our globe.
IN the spring of the year 1773 a singular rumour, soon fol-
lowed by a strange panic, obtained in Paris and rapidly
spread throughout France. A comet was shortly to appear
upon the earth's track, to come into collision with our planet,
and thus infallibly bring about the end of the world. The
origin of this rumour was a memoir which Lalande was to
have read before the Academy of Sciences on April 21; it
was, however, not read, but the title alone was sufficient to
create a popular ferment. The work of the learned astronomer
was entitled Reflexions sur les Cometes qui peuvent approcher de
la Terre. It was speedily imagined, and without the smallest
foundation — for nothing of the kind was to be found in the
memoir — that a comet predicted by the author was about to
dissolve the earth on May 20 or 21, 1773.
So great was the panic that Lalande, before publishing his
work, caused the following announcement to be inserted in the
Gazette de France of May 7 : ' M. de Lalande had not time to
read a memoir on the subject of the comets, which by their
approach to the earth may occasion disturbance to it ; but he
465
THE WORLD -OF COMETS.
observes that it is not possible to fix the dale of these events.
The next comet whose return is expected is that which is due in
eighteen years, but it is not amongst the number of those \vhich
can harm the earth.' This notice, it appears, did not allay the
public uneasiness, for, under the date of May 9, we read the
following in the Memoires de Bachaumont: —
' The cabinet of M. de Lalande is still besieged by the
curious, anxious to interrogate him upon the memoir in
question, and doubtless he will give to it a publicity which is
now necessary, in order to reassure those whose heads have been
turned by the fables to which it has given rise. So great has
been the ferment that some devots, as ignorant as they are
foolish, solicited the archbishop to have a forty hours' prayer,
in order to arrest the enormous deluge threatened ; and
that prelate was on the point of ordering the prayer, when
some Academicians made him sensible of the absurdity of such
a proceeding. The false announcement in the Gazette de
France has created a bad effect, for it is believed that the
memoir of the astronomer must have contained terrible truths,
since they were thus evidently disguised.'
We see that a century ago communiques were not more
efficacious than at the present day, and were just as much
believed. But the expressions which Bachaumont uses in
regard to the devots are not more misplaced than the abuse
which he finds means further on to lavish upon Lalande. How
much more to the point is the refined irony of Voltaire in his
Lettre sur la pretendue Comete ! Let the following short extract
speak for itself:—
'Grenoble. May 17, 1773.
'Some Parisians, who are no philosophers — and, if they are
to be believed, will not have time to become so — have informed
us that the end of the world is approaching, and that it will in-
fallibly take place on the 20th of this present month of May.
456
COMETS WHICH HAVE APPROACHED NEAREST TO EARTH.
* On that day they expect a cornet which is to overturn c ur
little globe and reduce it to impalpable powder, according to a
certain prediction of the Academy of Sciences which has not
been made.
'Nothing is more probable than this event; for James
Bernoulli, in his " Treatise upon the Comet," expressly pre-
dicted that the famous comet of 1680 would return with a
terrible crash on the 17th of May, 1719. He assured us that
its wig would signify nothing dangerous, but that its tail
would be an infallible sign of the wrath of heaven. If James
Bernoulli has made a mistake in the date, it is probably by no
more than fifty-four years and three days.
4 Now, an error so inconsiderable being looked upon by all
mathematicians as of no account in the immensity of ages, it
is clear that nothing is more reasonable than to expect the end
of the world on the 20th of the present month of May, 1773,
or in some other year. If the event should not happen, what
is deferred is by no means lost.
' There is certainly no reason to laugh at M. Trissotin when
he says to Madame Philaminte (Femmes Savantes, act iv.,
scene 3) : —
Nous 1'avons en dormant, madame, echappe belle :
Un monde pres de nous a passe" tout du long,
Est chu tout au travers de notre tourbillon ;
Et, s'il cut en chemin rencontre notre terre,
Elle cut e"te brisee en morceaux conime verre.
' There is no reason whatever why a comet should not
meet our globe in the parabola which it is describing; but what
then would happen? Either the force of the comet would be
equal to that of the earth or it would be greater or less. If
equal, we should do it as much harm as it would do us,
action and reaction being equal ; if greater, it would take us
along with it; if less, we should take it along with us.
457
THE WORLD OF COMETS.
* This great event can be managed in a thousand ways,
and no one can affirm that the earth and the rest of the planets
have not experienced more than one revolution from the em-
barrassment of a comet encountered in its way.'
Lalande's memoir was published in the course of the
year 1773. It appeared moreover in the Comptes rendus of the
Academy, and the prediction which had never been made was
soon forgotten. ' The Parisians will not desert their city,' says
Voltaire, in concluding his letter; ' they will sing their chansons,
and the " Comet and the End of the World " will be played at
the Opera Comique.'
What, then, after all, was the purpose of Lalande's work ?
To find by calculation the distances of the nodes of sixty-one
comets from the earth's orbit, as well as the distances of these
comets from the ecliptic, when the comet's radius vector was
equal to unity. By means of these elements it was possible
to determine which among known comets could most nearly
approach the earth, and, consequently, occasion or undergo the
greatest perturbations. The table which he gave was perfected
by a Swedish astronomer, Prosperin. The following extract
contains the most curious of the results : —
Comets which have Approached Nearest to the Earth.
Minimum distance be-
tween the comet and the
Date of arrival at the nearest
earth's orbit
points
In radii
In miles
Earth
Comet
Comet of 1472
0-0434
3,980,000
January 19
January 22
1680
0-0053
480,000
December 22
November 20
1684
0-0092
850,000
June 18
June 29
1702
0-0304
2,780,000
April 22
April 20
1718
0-0449
4,110,000
January 27
January 10
1742
00141
1,290,000
November 9
December 13
1760
0-0536
4,910,000
' January 16
December 31
1770
0-0183
1,680,000
July 1
July 1
458
COMETS WHICH HAVE APPROACHED NEAREST TO EARTH.
Of these comets two have approached the orbit of the earth
t6 a distance less than the hundredth part of the distance of
the earth from the sun, viz. the comets of 1680 and 1G84.
The first passed within 500,000 miles, and the second within
850,000 miles of the earth's orbit ; but both these comets were
in reality much more distant from the earth, as it was not at
the time of these passages at the nearest point of its orbit.
This was not the case, however, as regards the comet of 1770,
which passed on the same day as the earth through a point
only 168,000 miles distant from the spot occupied by our
globe five hours later.
SECTION II.
COMETS AND THE END OF THE WORLD.
Prediction of 1816 ; the end of the world announced for July 18 — Article in the
Journal dee Debats — The comet of 1832; its rencontre with the orbit of the earth —
Notice by Arago in the Annuaire du Bureau des Longitudes — Probability of a
rencontre between a comet and the earth — The end of the world in 1857 and the
comet of Charles V.
THE terrors of the year 1773 create a smile at the present day;
but similar fears, it ought not to be forgotten, have been
renewed from time to time in our own century ; as, for ex-
ample, in 1816, 1832, and 1857.
In 1816 a report was current of the approaching end of the
world ; July 18 was the date fixed for the fatal event. Some
days after there appeared in the Journal des Debats a satirical
article by Hoffmann, in which that critic ridiculed in the fol-
lowing manner the hypothesis * of the earth coming into colli-
sion with a comet : —
' A great mathematician (Laplace), to whom we owe the
complete exposition of the system of the world, and whose
work is law, has been kind enough to reassure us a little con-
cerning the uncivil comets of Lalande ; but that he is far from
having banished all cause of alarm we may judge from the fol-
lowing passage, which I will literally transcribe : " The small
* The quotation is taken from a curious little work of M. Maurice Champion,
La fin da, Monde et les cometes au point de vue histonque et anecdotique. Paris,
1859.
460
COMETS AND THE END OF THE WORLD.
probability of such a rencontre may, by accumulating during a
long series of years, become very great." Now, for many
centuries no comet has come in contact with our globe.
.... [Here follows an enumeration of the effects produced by
the collision of a comet, which we shall reproduce further on
from Laplace.] .... 'As it is a very long time,' continues
Hoffmann, ' since this catastrophe has taken place, and as the
possibility of the disaster increases with time, according to
our great mathematician, it seems to me prudent to set about
arranging our affairs; for, in three or four thousand years at
the latest, we shall see a new representation of this great
tragedy.'
In France wit, which in this case is the flower of good
sense, never fails to assert its rights, as Bayle and Voltaire have
already proved. Hoffmann in 1816 furnishes new testimony of
the same truth ; but not the less slow are superstition s^ ideas
to relax their hold, and, assisted by the popular ignorance of
astronomy, we see that the fear of comets, so vivid in the
Middle Ages, continues to reappear from time to time amongst
all classes of the people. There is, nevertheless, an essential
difference between the superstitious beliefs of former times and
the credulity of the present day ; in former times every appari-
tion of a comet passed for a kind of supernatural event, a
warning from above, and the fatal consequences arising from
the passage of the terrible visitor were so many decrees of
Providence. In our day a comet is more especially feared from
the widely-spread impression that a fortuitous meeting between
a comet and the earth is a fact which may arise in the natural
order of events. It may also happen, as in 1773, that a scien-
tific announcement wrongly interpreted gives rise to chimerical
fears, which find in ignorance and the remains of mystic beliefs
elements favourable to their propagation. In proof of which
we subjoin the two following examples.
461 '
THE WORLD OF COMETS.
The first of these is afforded by Biela's comet of six years
and three-quarters period, owing to the astronomical prediction
of its passage for the year 1832.
Olbers had just given the elements and the ephemeris of
the comet discovered in 1826 by Biela, and calculated by
Damoiseau to return in 1832. On October 29, before mid-
night, the new comet was expected to pass through its node
_ that isto say, to cut the plane of the ecliptic — at a point
a little within the earth's orbit, the distance of the node
' C. 29 OCt- 18SS
T. so nor. ISM. ~
Fig. 71.— The Orbit of the Earth and that of Biela's Comet in 1832. Relative positions
of the two bodies.
from the orbit, in fact, not exceeding 4*66 radii of our
globe. Now, four radii and two-thirds are equivalent to rather
less than 18,600 miles ; so that however insignificant the
dimensions of the nucleus and coma — according to the obser-
vations made by Olbers in 1805 the diameter of the atmosphere
was equal to about 5 radii — it was evident that on October 29
the terrestrial orbit would intersect some portion of the atmo-
sphere of the comet.
Nothing more was needed, these details having transpired,
462
COMETS AND THE END OF THE WORLD.
to originate the report of an approaching rencontre between the
comet and the earth. Our globe, violently struck, would be
shattered to pieces; the end of the world was evidently at
hand. One point alone had been forgotten by the alarmists,
and Arago, who undertook to draw up for insertion in the
Annuaire du Bureau des Longitudes a notice which should be
calculated to allay the public uneasiness, alludes to it in the
following terms. After concluding, as we have seen above,
' that on the 29th of October next a portion of the orbit of the
Fig. 72. — Biela's Comet at its node, October 29, 1832. Supposed position of the Comet
at its least distance from the Earth.
earth will be comprised within the nebulosity of the comet,' the
illustrious astronomer continues thus : —
' There remains but one more question, which is this : at
the time when the comet is so near our orbit that its nebu-
losity will envelope some portion of it, where will the earth itself
be situafed ?
' I have already said that the passage of the comet very
near to a certain point of the terrestrial orbit will take place on
the 29th of October, before midnight; the earth, however, only
arrives at this point on the morning of the 30th of November,
463
THE WORLD OF COMETS.
that is to say, more than a month later. We have, then, only
to remember that the mean velocity of the earth in its orbit is
1,675,000 miles per day, and a very simple calculation will
suffice to prove that the comet of six and three-quarter years
period will be, at least during its apparition in 1832, always
more than 49 millions of miles from the earth T
Arago did well to reserve the question of future apparitions;
for this same comet of 1832, forty years later, if it did not
come into collision with the earth, must at least have grazed its
surface.
Finally, before leaving these imaginary rencontres of comets
and the end of the world, let us remark that the same chimerical
fears were current in Europe in 1857, a propos of the predicted
return of the comet of Charles V. This time the mystification
came from Germany ; according to a fantastic prediction the
world was to be destroyed by fire, burnt by the terrible comet
of June 13, in the year of grace 1857 ! The prediction at first
related to the rencontre of an imaginary comet only; but after-
wards the serious expectancy with which astronomers awaited
the return of the comet of 1264 and 1556 suggested the idea
of attributing the future catastrophe to the expected comet,
although nothing in the elements of its orbit justified the pro-
bability of such a rencontre.*
In his notice upon the comet of 1832 Arago raises the
following question : Can a comet come into collision with the
earth or any other planet? By reasoning based upon the fact
that the orbits of comets intersect the heavens in all directions,
that they constantly traverse our solar system, and penetrate
* Perhaps it had been ascertained that the comets of 1264 and 1556, in
Lalande's table, approached the earth's orbit to about 0*08 of the distance of the
earth from the sun ; and that the earth and the comet passed on the same day
(March 12, 1556) through the points of their nearest mutual approach — at
7,200,000 miles, however, from each other.
464
COMETS AND THE END OF THE WORLD.
to the interior of the orbits of the planets, even to the regions
comprised between Mercury and the sun, he comes to the con-
clusion ' that it is not at all impossible for a comet to encounter
the earth.' But, after having stated the possibility of the fact,
he hastens to examine its probability. For this purpose he
supposes a comet of which nothing more is known than that at
its perihelion it will be nearer to the sun than we are ourselves,
and that its diameter is equal to a fourth part of the earth.
By the theory of probabilities Arago then finds that the odds
are 281 millions to one against a rencontre with the earth.
This probability, it is true, should be increased at least tenfold
if, instead of our rencontre with the cometary nucleus, we were
to substitute the entire nebulosity, the volume of which is
comparatively much more considerable.
To illustrate the significance of the numerical results to
which considerations of this kind lead, Arago continues : ' Let
us for a moment suppose that if a comet were to strike the
earth with its nucleus, it would annihilate the whole of the
human race. The danger of death to which each individual,
in that case, would be exposed from the apparition of an un-
known comet would be exactly equal to the danger he would
incur, supposing that in an urn containing a total number of 281
millions of balls there should be only one white ball, and that
his condemnation to death were the inevitable consequence of
this same white ball presenting itself at the first drawing.
Anyone who will consent to use his reason, however attached
to life he may be, will laugh at so trilling a danger ; and yet
the moment that a comet is announced, before it has been
observed or its course determined, it is for each individual of
our globe the white ball of the urn above-mentioned.'
These calculations are indisputably correct as regards their
general application. But, when a particular comet is in ques-
tion, whose elements are known, all considerations of probability
405 , H II
THE WORLD OF COMETS.
are out of place. Arago with reason calls attention to this fact
in reference to the comet of 1832, and its apparition in that
year. With respect to ulterior apparitions it is different.
There is always some uncertainty ; the orbit of the comet and
its elements may be modified by the planetary perturbations,
and the return of the comet to its node may take place so that
both the earth and the comet may arrive, if not at the same
point together, at least sufficiently near one another to cause the
contact of some of their parts. We have already said that this
is what probably took place in 1872, as regards the comet of
1832, during the night of November 27 The meeting was
absolutely inoffensive.
466
SECTION III.
MECHANICAL AND PHYSICAL EFFECTS OF A COLLISION WITH
A COMET.
Opinions entertained by astronomers of the last century: Gregory, Maupertuis,
Lambert — Calculations of Lalande ; comets move too rapidly in the vicinity of the
earth for the effects of their attraction to come into play — Opinion of Laplace — •
The collision of a comet with the earth ; its effect according to the mechanical
theory of heat.
IT is interesting to note the opinions formed by savants a cen-
tury ago respecting the probable effect of a collision between
a comet and the earth. Further on we shall speak of the
theological romance invented by Whiston for the scientific ex-
planation of the Deluge. According to Whiston the famous
comet of 1680, after having, 4000 years ago, produced the
universal deluge, is destined to accomplish the destruction of
the world, and our globe will be ultimately set on fire by the
same comet which had previously inundated it.
Whiston wrote at the end of the seventeenth century. In
the middle of the eighteenth century theological speculations
engaged but very slightly the attention of astronomers ; but
that a very exaggerated idea continued to prevail respecting
the amount of injury which the proximity of a comet or
its collision with the earth would be capable of producing
is undoubted.
In 1742 Maupertuis, in his Lettre sur la Cimete, writes as
467 ' H n 2
THE WORLD OF COMETS.
follows : ' With their variety of movements it is clearly possible
for a comet to encounter some planet or even our earth upon
its way; and it cannot be doubted that terrible results would
ensue. On the mere approach of these two bodies great
changes would be effected in their movements, arising either
from their mutual attraction or from the action of some fluid
compressed between them. The least of these movements
would suffice to change the position of the axis and the poles
of the earth. That portion of the globe which had been
previously near the equator would be situated, after such an
event, near the poles, and that which had been near the poles
would be situated near the equator.
' Some comets passing near the earth,' he observes else-
where, ' might so alter its movement as to cause it to become
a comet itself. Instead of pursuing its course, as it now
does, in an uniform region, having a temperature suitable to
man and the animals which inhabit it, the earth, exposed to
the greatest vicissitudes, sometimes scorched at its perihelion,
sometimes frozen by the cold of the remotest regions of the
heavens, would be perpetually passing from one extreme to
another, unless some comet should again come near enough to
change its course and re-establish it in its original uniformity.'
Lastly, some large comet might, if Maupertuis is to be be-
lieved, divert the earth from its present orbit and subject it
to its own attraction ; in a word, make a satellite of our globe,
\vhich, henceforth compelled to follow the movements of the
comet, would be exposed to the same vicissitudes as on the
preceding hypothesis. ' A comet could in like manner de-
prive us of our moon, and if nothing more happened to us
we should have no reason to complain. But the severest
accident of all would be if a comet were to come in contact
with the earth and break it into a thousand pieces. Both
bodies would doubtless be destroyed; but from the fragments
468
EFFECTS OF A COLLISION WITH A COMET.
gravity would speedily form a new planet or several new
planets.'
No one could be more accommodating. Maupertuis, it
is plain, is of the same way of thinking as the mathe-
matician so amusingly described in the Lettres Persanes, who
regarded natural accidents of the most disastrous kind and
the most formidable catastrophes simply as matters for calcu-
lation and opportunities for the furtherance of scientific know-
ledge.
So much for the mechanical effects due to the mass of a
comet, which Maupertuis evidently regards as of an order
of magnitude comparable to that of the earth. Now for
the physical effects.
4 The approach of a comet,' he observes, ' might be at-
tended with results yet more fatal. I have not yet spoken to
you of the tails of comets. These have been the subject of
opinions not less curious than comets themselves. According to
the most probable conjecture they are immense torrents of ex-
halations and vapours driven forth from the body of the comet
by the heat of the sun. The strongest proof of this is that
these appendages make their appearance only when the comet
has approached to within a moderate distance of the sun ; that
they increase in proportion as the comet draws near to him,
and diminish and become dispersed as it withdraws.
*A comet accompanied by a tail might pass so near the earth
that we should find ourselves drowned in that torrent which
it draws along with it, or in the atmosphere of the same nature
which surrounds it. The comet of 1680, which approached
near the sun, was subjected to a heat twenty-eight thousand
times greater than that which the earth experiences in summer.
Mr. Newton, from different experiments which he has made
respecting the heat of bodies, having calculated the degree of
heat which the comet would have acquired, finds that it must
469 /
THE WORLD OF COMETS.
have exceeded two thousand times that of red-hot iron, and
that a mass of red-hot iron as large as the earth would re-
quire 50,000 years to cool. What, then, could have been the
heat which still remained in the comet when, coming from the
sun, it crossed the orbit of the earth? Had it passed nearer,
it would have reduced the earth to ashes or vitrified it ; and
if only its tail had reached us, the earth would have been
inundated by a river of fire, and all its inhabitants killed —
scalded to death like a colony of ants in boiling water.'
This, however, is the bad side of the rencontre. For, ac-
cording to Maupertuis, who is afraid of having been too severe
upon comets, they are also able to procure for us certain
advantages ; to raise, for instance, the axis of our globe, now
too much inclined, and ' to maintain the seasons at a per-
petual spring; to diminish the eccentricity of the orbit, and so
cause a more equable distribution of light and heat. Lastly,
instead of taking away our moon, it might itself be con-
demned to revolve about our earth, to illuminate our nights; to
become to us, in short, a second moon. Who knows but that
in former times we obtained in this manner possession of our
own moon, originally, perhaps, some little comet, which, in
consequence of having too nearly approached the earth, has
been made captive by it? '
This opinion, which Pingre quotes, is, in his estimation,
all the more probable, as it is based upon a tradition univer-
sally spread amongst the Arcadians. According to the testi-
mony of Lucian and Ovid these people believed themselves to
be more ancient than the moon. But the physical constitution
of our satellite is altogether different from that of known comets,
and Arago with reason calls attention to the fact that ' the
almost total absence of atmosphere about the moon, far from
being favourable, is rather adverse to the opinion which sup-
poses the moon to be an ancient comet.'
470
EFFECTS OF A COLLISION WITH A COMET.
Before leaving the speculations of Maupertuis let us quote
the following passage from his letter, which rhows the ideas
then current respecting the physical and chemical constitution
of comets, and the nature of the substances of which they are
composed : —
' However dangerous might be the shock of a comet, the
comet itself might, nevertheless, be so small as to be fatal
only to that part of the earth with which it happened to come
in contact, so that we might, perhaps, be compensated for the
destruction of some kingdom by the enjoyment which the
rest of the world would derive b}7 the rarities which a body
coming from so vast a distance could not fail to bring with it.
The debris of those masses which we despise might prove, to
the surprise of everyone, to be formed of gold and diamonds.
But who would be the most astonished, ourselves or the in-
habitants precipitated by the comet upon our earth? What
should we think of each other?'
After all, the idea of Maupertuis is not so strange as
might be thought. If no very small comet has yet fallen
upon the earth, we frequently receive debris which have be-
longed to some celestial body; if neither gold nor diamonds
fall upon the earth, it is certain that other minerals fall, iron
and nickel, for example ; but this is a matter that relates
rather to shooting stars.
Gregory, a learned astronomer of the eighteenth century,
observes, in his work Astronomice physicce et geometricoe
elementa (liv. v. prop. iv. coroll. 2) : 'If the tail of a comet
should extend as far as our atmosphere, or if a part of the
sparse matter in the heavens of which that tail is composed
should fall under the influence of our attraction, the exha-
lations of which it is formed, by mixing with the air that
we respire, would occasion changes particularly sensible to
animals and plants. Vapours, indeed, brought from distant
471
THE WORLD OF COMETS.
and foreign regions, and excited by an intense heat, would be
fatal, perhaps, to living beings upon the surface of the earth,
and give rise to events similar to those which the testimony of
all nations and universal consent concur in representing as a
consequence of the apparition of comets, and which it does
not become philosophers to assume too hastily to be ridicu-
lous fables.'
Further on we shall return to examine the justice of these
and similar views on the subject of cometary influences. Let
us at the present moment confine ourselves to the opinions of
astronomers concerning the probable effect of a mutual ap-
proach or rencontre.
Lambert, in his Lettres Cosmologiques (1765), thus expresses
himself on the subject of these effects: —
' If comets,' he observes ' produce neither war, nor famine,
nor mortality, nor the fall of empires, what are these evils in
comparison with the catastrophes with which they menace
the entire globe ? When we consider the movement of these
bodies, arid reflect upon the law of gravitation, it is not diffi-
cult to perceive that their near approach might be the occasion
of the most dire catastrophes to our earth, cover it again with
the waters of a deluge or cause it to be consumed by fire, crush
it to atoms, or at least divert it from its orbit, carry away its
moon, or worse still, carry away itself, and, bearing it off into
the regions of Saturn, compel it to endure a winter of several
centuries, which neither man nor animals would be capable of
resisting.' This passage suffices to show that Lambert shares
the ideas of Maupertuis respecting the probable influences of
comets. But these events, though possible are not in his
opinion probable, and he founds his opinion upon considerations
based upon final causes, upon the order and harmony of the uni-
verse, the necessity for the preservation of those vast bodies,
whose duration must needs be proportional to their masses.
472
EFFECTS OF A COLLISION WITH A COMET.
He even considers * that all these bodies have exactly the
mass, the weight, the position, the direction and velocity
requisite, to enable them to avoid dangerous collisions. A
comet, for example, passing in the near vicinity of Jupiter,
might be turned aside by that great planet, either to the right
or left, with the express design of preventing one of these
rencontres. This again is purely imaginary, and entirely
opposed to the facts. The comet of Biela has been shat-
tered; suns, like the stars of 1572, 1664, and 1866, have
blazed forth suddenly, and have been extinguished almost as
suddenly, under the eyes of observers. Revolutions and catas-
trophes happen in the physical world, which they seem to dis-
turb for a moment as in the social world; so far from being
" o
departures from natural laws, science, which establishes their
existence, has no other task than to show how they result from
them.
The following calculation has been made by Lalande : ' If
a comet * were five or six times nearer to us than the moon,
that is to say, if it were passing at a distance of 1 3,000 leagues
(40,000 miles) from the earth, it would suffice to raise the
waters of the sea two thousand fathoms above the ordinary level,
which would, in all probability, submerge the continents of the
four quarters of the world.' To this calculation, Dionys du
Sejour makes an objection, which destroys its value. In virtue
of a principle, demonstrated by D'Alembert, he shows that ' if
we suppose the globe entirely covered with water, to the depth
of one league, the comet would occupy 1en hours and fifty-two
minutes in producing its effect, whatever that might be, upon
the tides; this duration would depend neither upon the size,
nor density, nor proximity of the cornet, but solely upon the
depth of the fluid. If the fluid were two leagues deep, the
[* The mass of the comet appears to be supposed equal to that of the earth.
—ED.]
473
THE WORLD OF COMETS.
elevation of the waters would continue for eight hours, twenty-
five minutes, eleven seconds. Now, the action of the comet
upon any one portion of the sea is far from being of so long a
duration. Let the comet, at its perigee, be 13,000 leagues from
the earth, one hour after it will be at least at a distance of
16,549 leagues, and will be vertically over a point of the earth,
23° 14' distant from the place it occupied at the time of peri-
gee. At the end of the second hour, the point to which it
corresponds will have changed 27° 36', and the distance of
the comet from the earth will be 24,768 leagues. We have
here chosen the most favourable case possible for the action of
the comet. On another hypothesis, in one half hour only, the
comet would be removed 32,569 leagues from the earth, and its
corresponding point upon the earth would have changed 81°
27' 30''.' All other possible hypotheses give a result inter-
mediate to these two extremes. We may judge, from these
considerations, whether comets have time to produce the great
disturbances in the tides, which they would produce if they re-
mained for a longer time vertically over the same point of the
sea.
This objection has an important bearing upon the question
now before us; for it does away entirely, so to speak, with the
danger of a very near approach between a comet and the
earth. This caused Laplace to say, thus confirming the con-
clusions of Dionys du Sejour : ' Comets pass with such rapidity
in the vicinity of the earth, that the effects of their attraction
need occasion no alarm. It is only by striking against the
earth that they can be the cause of serious or fatal injury to
our globe.'
The illustrious author of the Mecanique Celeste does not
regard a collision as impossible, but he considers the pro-
bability of such an event as extremely slight. Although he
points out that the cometary masses, as indicated by their in-
474
EFFECTS OF A COLLISION WITH A COMET.
appreciable influence upon the planetary movements, are so
small that in all probability only local disturbances would be
produced; still he has depicted in the following terms the
effects of a rencontre, on the supposition that the mass of the
comet were comparable to that of the earth (this is the passage
referred to in Hoffmann's article).
' The axis and the movement of rotation would be
changed; the seas would abandon their ancient positions, in
order to precipitate themselves towards the new equator ; a
great portion of the human race and the animals would be
drowned in the universal deluge, or destroyed by the violent
shock imparted to the terrestrial globe; entire species would be
annihilated; all monuments of human industry overthrown;
such are the disasters which the shock of a comet would produce,
if its mass were comparable to that of the earth' (Exposition
du Systeme du Monde). Laplace, moreover, does not seem far
from believing that such a catastrophe has taken place, and the
geological revolutions, the cataclysms which the ideas of
Cuvier, then dominant, tended to refer back to a not very re-
mote date, appear to him capable of being explained by such an
event. 4 We see then, in effect,' he continues, ' why the ocean
has receded from the high mountains, upon which it has left
incontestable marks of its sojourn. We see how the animals
and plants of the south have been able to exist in the climate
of the north, where their remains and imprints have been dis-
covered; in short, it explains the newness of the moral
world, certain monuments of which do not go further back
than five thousand years. The human race reduced to a small
number of individuals, and to the most deplorable state, solely
occupied for a length of time with the care of its own preser-
vation, must have lost entirely the remembrance of the
sciences and the arts ; and when the progress of civilisation made
475
THE WORLD OF COMETS.
these wants felt anew, it was necessary to begin again, as if
man had been newly placed upon the earth.'
Laplace, at the present day, would discard this explanation
of the geological facts of past times, which modern science has
since very differently interpreted. But it is interesting to
know the opinion of the great mathematician with regard to
the probable results of a collision between a comet and the
earth ; an opinion differing very little from that of the sa-
vants of the eighteenth century, and consequently very far re-
moved from that of certain contemporary astronomers, who,
like Sir John Herschel and Babinet, regard comets as visible
nonentities.
47(1
SECTION IV.
CONSEQUENCES OF A COLLISION BETWEEN A COMET AND THK
EARTH ACCORDING TO THE MECHANICAL THEORY OF HEAT.
THE mathematicians and astronomers who have alluded to the
effects of a collision between our earth and a comet, have
more especially considered the event from a mechanical point
of view ; the two bodies were for them simply two projectiles
which, both animated with enormous velocities, could not fail
to encounter each other with a violence dependent upon their
respective masses, velocities, and directions of motion. They
foresaw only the dislocation or rupture of two gigantic masses,
a catastrophe which would inevitably cause the destruction
of the human race, and of all living beings upon the surface of
the earth.
Some philosophers, believing a comet to be an incandescent
mass, or at least to have become heated to an intense degree
during its passage in the near vicinity of the sun, have con-
ceived that it would inevitably set fire to our globe ; in which
case Ave should perish both by the shock and by lire.
But it was not then possible to view the phenomenon in its
true light, since the great principle of the conversion of me-
chanical energy* into heat had not at that time been dis-
[* The kinetic energy, or vis viva, of a body in motion is measured by the
product of its mass and the square of its velocity. There are different forms of
477
THE WORLD OF COMETS.
covered. Let us therefore continue the same hypothesis of a
comet of solid nucleus, of a mass comparable to that of our
globe, and coming into collision with the earth from any direc-
tion whatever. The maximum effect, it is clear, would take
place if the two bodies, travelling in opposite directions, were to
encounter each other, so as to annihilate their respective motions,
which could only happen in the event of the masses and velo-
city of the two bodies being equal. Under other conditions
the result would vary as regards the amount of the effects pro-
duced, but not as regards their nature.
Now, the new principle, which has been established both
by experiment and theory, is that even should all movement
be apparently annihilated by the shock, it is in reality integrally
preserved; only, it is transformed by molecular movement into
heat.
The comet and the earth, by coming into collision with each
other as we have just supposed, would be therefore stopped in
their movements about the sun, and the sum of the energies of
their motions, at the moment of collision, would be entirely
converted into heat. Now, to show the enormous amount of
heat that would be generated by the mere stoppage of the
earth, we shall quote the following passage from Tyndall : —
' Knowing, as we do, the weight of the earth and the velo-
city with which it moves through space, a simple calculation
enables us to determine the exact amount of heat that would
be developed, supposing the earth to strike against a target
strong enough to stop its motion. We could tell, for ex-
energy, of which heat is one ; and the principle of the conservation of energy
asserts that the total energy of a system of bodies cannot be increased or dimi-
nished by any mutual action between them, although it may be transformed into
any other form of which energy is susceptible. Thus, when a moving body ia
brought to rest, the kinetic energy of its motion is transformed into some other
form of energy, and is, in fact, expended in the generation of a definite quantity
of heat. — ED.]
478
CONSEQUENCES OF A COLLISION.
ample, the number of degrees which this amount of heat would
impart to a globe of water equal to the earth in size. Mayer
and Helmholtz have made this calculation, and found that the
quantity of heat which would be generated by this colossal
shock would be quite sufficient, not only to fuse the entire
earth, but to reduce it in great part to vapour.'
The catastrophe, as we thus see, would not be less enormous
than was at first supposed. The two bodies, by their collision,
would be converted into one mass, one part of whose elements
would be in a state of fusion, whilst the rest would form an
envelope of vapour. Nothing could more nearly resemble the
idea we form at the present day respecting certain comets ; but
on our hypothesis all movement of translation would be anni-
hilated. The new star, abandoned to the influence of the solar
gravitation, would necessarily fall upon the surface of the sun,
and the amount of heat generated by the blow, would be equal
to that developed by the combustion of 5,600 globes of solid
carbon, each having a volume double that of the earth.
Thus, even when considering the matter from the same
point of view as the savants whose opinions have been quoted
in the preceding section, we arrive at very different results.
But it must be remembered that it is all but certain that comets
have masses so very inferior to that of the earth, and that their
physical constitution is so different, that a meeting would have
little resemblance to a blow or shock such as would result from
the collision of two solid globes.
479
SECTION V.
THE COMET OF 1680, THE DELUGE, AND THE END OF
THE WORLD.
Ancient apparitions of the comet of 1680, on the hypothesis of a revolution of 575
years — Their coincidence with famous events — Whiston's theory of the earth : our
globe is an ancient comet, whose movements and constitution have been modified
by comets — The catastrophe of the Deluge caused by the eighth anterior apparition
of the comet of 1680 — Final catastrophe : burning of the earth — Future return
of our globe to the condition of a comet.
THE comet of 1680 is one which has been made the subject of
considerable discussion. If we adopt the calculations of Halley,
confirmed in the first instance by Newton, it would be that
which appeared in the years 531 and 1106 of our era, announced
,in the year B.C. 43, the death of Cassar, presided at the taking
of Troy, and eleven or twelve centuries earlier, was the direct
cause of the deluge mentioned in the Mosaic records.
In 1106. the apparition of this celebrated comet did not, it
is true, coincide with any great historical event; but, according
to the chroniclers, it was of great brilliancy, ' resembling a
flaming torch, covering with its rays a great portion of the
heavens, and filling all minds with terror.' 575 years before,
that is to say in 531, ' during twenty days was seen to the
westward, a very large and fearful comet ; it extended its rays,
that is to say its tail, towards the most elevated portion of the
heavens; on account of its resemblance to a burning lamp
480
THE COMET OF 1680 AND THE KAUTII.
it received the name of Lampadias ' ( Theophanes, C/iron.,
quoted by Pingre). Other apparitions would have taken place
in the year B.C. (519, that is, at the date of the destruction of
Nineveh; and again in B.C. 1769, or, following Freret, under
the reign of Ogyges, who, according to the Greek legends, was
contemporary with another deluge.
Whiston, an Englishman of the eighteenth century, a con-
temporary of Newton, and alike theologian and astronomer,
published in 1696, ' A New Theory of the Earth,' in which he
proposed to explain, by the action of a comet, the geological
revolutions recorded in the Book of Genesis. His theory was
at first entirely hypothetical, and not framed with reference to
any comet in particular ; but when Halley assigned to the
famous comet of 1680 an elliptic orbit, with a period of 575
years, and Whiston identified the dates of two of its former
apparitions with the years 2344 and 2919, that is to say, with
two of the years fixed by chronologists as the date of the
Mosaic deluge, he hesitated no longer ; he developed his theory,
and made the comet of 1680 appear, not only as the destroyer
of the human race by flood, but also as the destroyer of the
world by fire in future ages.*
Let us briefly give an idea of this curious theory, confining
ourselves at present to that part of the hypothesis which con-
cerns the deluge.
According to Whiston, the earth is an ancient comet whose
perihelion was formerly very near the sun ; the excessive tem-
perature to which the comet was subjected at each of its peri-
helion passages explains the central, and continually main-
* The 28th of November, 2349, or 2348, according to the modern Hebrew
text; December 2, 2926, according to the Samaritan text; the year 3308
according to others, would correspond to the exact chronological date of the
Biblical deluge. Between 2349 and 2926 there are 577 years, only two years
more than the period calculated by Halley for the comet of 1 680.
481 ' I I
THE WORLD OF COMETS.
tained, heat of the globe. When the earth was to be rendered
habitable, one operation alone sufficed; the centrifugal force of
the comet was diminished, and its orbit thus rendered less
eccentric; this transformation effected, the eccentricity, never-
theless, was still sufficiently great to allow the hemisphere,
destined to serve as the dwelling-place for man and animals, to
enjoy the presence of the sun for a period of nine or ten months.
Thanks to these changes, the thick atmosphere of the ancient
comet became purified, and the air, the soil, and the water
gradually found their equilibrium. The sun and the moon
looked down upon the earth, and man and animals appeared.
When man had sinned, a small comet, passing very near
the earth, cutting obliquely the plane of its orbit, impressed
upon it a movement of rotation. No doubt we must attribute
to this same comet the perfect circularity of the terrestrial
orbit which, according to Whiston, was the case before the
deluge. Pingre remarks, in citing Whiston, that : ' God had
foreseen that man would sin, and that at length his crimes
would demand a terrible punishment ; consequently, he had
prepared from the beginning of the creation a comet which
he designed to make the instrument of his vengeance. This
comet is that of 1680.' How was the catastrophe accomplished?
Briefly as follows, according to Whiston.
On Friday, November 28, 2349, or on December 2, 2926,
the comet was situated at its node, and cutting the plane of the
earth's orbit at a point from which our globe was separated by
a distance of only 3,614 leagues, of twenty-five to a degree. The
conjunction took place at the hour of noon under the meridian
of Pekin, where Noah, it appears, was dwelling before the
flood. Now, what was the effect of this comet, to which Whiston
assigns a mass equal to a quarter of the earth's mass ? It
caused a prodigious tide, not only in the waters of the sea, but
also in those underneath the solid crust, disrupted by the move-
482
THE COMET OF 1080 AND THE EARTH.
ment of rotation, and in the densest parts of the fluid portion of
the terrestrial nucleus. The mountain chains of Armenia, the
Gordian mountains, which were nearest to the comet at the
moment of conjunction, were cleft and shaken to their founda-
tions, and thus ' were all the fountains of the great deep broken
up.' The disaster did not end here. The atmosphere and the
tail of the comet coming in contact with the earth and its
atmosphere, loaded the latter with aqueous and terreous par-
ticles, and ' thus were opened all the cataracts of heaven.'
The depth of the waters of the deluge was, according to
Whiston, six English miles, one mile of which was due to the
eruption of the interior fluid, about five miles to the atmosphere
or coma of the comet, and some little to its tail.
It was, then, as we see, a real inundation, an universal
deluge which, according to this remarkable theory, was caused
by the passage of the comet of 1680 to its node in the close
vicinity of the position occupied by the earth, 4,223 years ago,
according to some, 4,800 years according to others. The
shock might perhaps have been sufficient to accomplish the
work of destruction, but unquestionably a depth of six miles
of water all over the earth was a more certain means of anni-
hilation.
Now, how is this comet, which in the first instance drowned
all the living beings upon the earth, to cause on its return the
destruction of the earth's inhabitants by fire? Whiston is
equal to the occasion. A second passage in the vicinity of the
earth, but behind or to the west of it, will retard the movement
of our globe, and change its nearly circular orbit into a very
eccentric ellipse. The earth, at the time of its perihelion pas-
sage, will be situated in close proximity to the sun ; it will
experience an intense degree of heat, and enter into com-
bustion.
But the comet may have a direct action as well ; it may
483 * i 2
THE WORLD OF COMETS.
meet the earth and strike against it. We know that the comet
of 1680 approaches very near to the surface of the sun, so that,
following Pingre^s summary of Winston's views, 'hardly can the
mouth of a volcano vomiting forth lava liquified by the interior
consuming heat give an idea of the fiery atmosphere of this
comet. The air will then interpose no obstacle to the activity
of the central fire ; on the contrary, the inflamed particles with
which our atmosphere will be charged, carried down by their
own weight into the half-open bowels of the earth, will power-
fully second the action of the central fire. This comet might
even separate the moon from the earth, and affect the diurnal and
annual motion of the earth by rendering both these movements
equal, and by destroy ing besides the eccentricity of the terrestrial
orbit, which would again become circular as before the deluge.
Lastly, after the saints have reigned a thousand years upon the
earth, itself regenerated by fire, and rendered habitable anew
by the Divine will, a comet will again strike the earth, the ter-
restrial orbit will be excessively elongated, and the earth, once
more a comet, will cease to be habitable.'
Such is the romance conceived by Whiston, a man of great
erudition and science, but who shared the fault of his time
in wishing to make his conceptions accord both with theology
and astronomy. We are here only concerned with the scientific
side of the question; and it is certain, and it was so a hundred
years ago, that Whiston's theory is untenable. We will only
notice two vital objections : in the first place, the enormous
mass we are compelled to assign to the comet of 1680, and
which no astronomer of our time would admit as probable ; in
the second place, even assuming such a mass, its action would
be of so short duration, by reason, as we have seen, of the rela-
tive velocities of the comet and the earth, that the supposed
effects would not have time to manifest themselves. But
geologists, we believe, would have other objections to make to
484
THE COMET OF 1080 AND THE EARTH.
an hypothesis which we have only recorded because it is cele-
brated in science; and because the part it assigns to comets is
truly curious.
A last and capital objection is this : The discussion of the
elements of the comet of 1680 made by Encke with more
accurate data than Halley possessed, has entirely overthrown
the supposed chronological coincidences with its anterior appa-
ritions. According to the new elements, the period of the
comet is not 170 years (Euler), nor 575 years (Halley), nor
5,864 years (Pingre), but 8,814 years.
485
SECTION VI.
PASSAGE OF THE EARTH THROUGH THE TAIL OF A COMET
IN 1861.
Possibility of our globe passing through the tail of a comet — Has such an event ever
taken place ? — The great comet of 1861 — Relative positions of the earth and one
of the two tails of that comet — Memoir of M. Liais and the observations of
Mr. Hind.
THUS far, in treating of the possibility of a rencontre between
n comet and the earth, we have more especially had in view
the nucleus, or rather that portion of the cornet's nebulosity
which constitutes the coma. The effects of the rencontre have
been studied on certain hypotheses respecting the mass and
physical constitution of the comet whose nucleus we have sup-
posed to be solid; this is far from certain, and, in any case,
seems to be exceptional, as it is only in certain comets that
the head is sufficiently condensed to exhibit a luminous
nucleus.
A rencontre, of much greater probability, is that which
would arise from the passage of the earth through the volu-
minous nebulosity of which the tail is formed. In all proba-
bility the masses of these appendages are all but inappreciable.
Whatever opinion we may form of their nature, whether we
regard them with Cardan and certain savants of our day as
purely optical effects without material reality, or see in them
the most tenuous portions of the atmosphere of the comet
486
THE TAIL ()F THE COMET OF 1861 AND THE EARTH.
projected by a repulsive force, it appears certain that they con-
sist of quantities of matter of extremely slight mass, and of even
less density. It would be ridiculous to speak of a shock or any
other mechanical effect; but it is not altogether evident that
the matter of a comet might not produce some perceptible
modification of the atmosphere of our globe.
Before considering what would happen in the event of the
earth passing through the tail of a comet, we are naturally led
to inquire if such an event has ever actually occurred. Now,
according to several contemporary astronomers, the earth was,
in fact, on June 30, 1861, plunged for some time in the nebu-
losity forming the large tail of the great comet of that year.
M. Valz, in giving the elements of the comet, has observed :
' It follows that the comet having passed its node on June 28,
at 9.50 p.m., at the distance of 0*132 from the orbit of the
earth, the latter being less than 2° in advance of the node,
must have been situated within the nebulosity of the tail,
which was itself in the plane of the ecliptic. M. Loewy, in
the Bulletin de V Observatoire of July 12, likewise observes :
' It is probable that about June 28, the earth touched the tail
of the comet.' M. Pape, of Berlin, was of a different opinion,
his calculations leading him to conclude that an interval of
more than two millions of miles separated the tail of the
comet from the earth ; but, according to M. Valz, this arises
from the German astronomer having estimated the apparent
breadth of the tail at 3°, whereas he himself estimated it at
6°, and Father Secchi at as much as 8°. M. Le Terrier, in giving
the elements calculated by M. Loewy and Mr. Hind, adds the
following remark : ' Did the earth pass through the tail of
the comet? This question, apparently so simple, is, in reality,
very complex. The calculations are complicated, and the data
fail to determine this point with certainty.'
That the earth did pass through the tail of the comet was
487
THE WORLD OF COMETS.
the opinion of Mr. Hind from the very first. The following is
an extract from the letter written on this subject by the Eng-
lish astronomer to the editor of the Times : —
' Allow me to draw attention to a circumstance relating to
the present cornet, which escaped my notice when I sent a
communication on the 3rd instant, but which is now possessing
some interest. It appears not only possible, but even pro-
bable, that in the course of Sunday last the earth passed
through the tail, at a distance of, perhaps, two-thirds of its
length from the nucleus.
' The head of the comet was in the ecliptic at 6 p.m. on
June 28, distant from the earth's orbit 13,600,000 miles on the
inside, its longitude, as seen from the sun, being 279° 1'. The
earth, at this moment, was 2° 4' behind that point, but would
arrive there soon after 10 p.m. on Sunday last. The tail of a
comet is seldom an exact prolongation of the radius vector, or
line joining the nucleus with the sun ; towards the extremity
it is almost invariably curved ; or, in other words, the matter
composing it lags behind what would be its situation if it
travelled with the same velocity as the nucleus. Judging
from the amount of curvature on the 30th, and the direction
of the comet's motion as indicated by my orbit already pub-
lished, I think the earth would very probably encounter the
tail in the early part of that day, or, at any rate, it was cer
tainly in a region that had been swept over by the cometary
matter shortly before.'
M. Liais, who observed the same comet in Brazil, speaks
with still more certainty. He bases his assertions upon obser-
vations of his own. made before and after the perihelion pas-
sage, upon the breadth and direction of the tail of the cornet,
as well as upon the elements of the orbit calculated by M.
Seeling. We shall not enter into the details of the calculation
and the discussion given by this savant in VEspace Celeste, but
488
THE TAIL OF THE COMET OF 18C,1 AND THE EAKTIf.
content ourselves with stating the results. According to him,
not only the earth, but the moon also, entered the tail of
the comet on the morning of June 30, and at 6.12 p.m. on
Fig. 73.— Passage of the Earth through the tail of the comet of 1861, on Juno 30.
that day our globe was plunged in it to a depth of 273,000
miles. Figures 73 and 74 give, the first, the position of the
Fig. 74. — Positions occupied by the Earth and the Moon in the interior of the second tail
of the comet of 1861.
comet in the plane of its orbit, at the moment of the passage
of the axis of the second tail across the terrestrial orbit ; the
second, a section of the tail perpendicularly to its axis. In
489
THE WORLD OF COMETS.
the latter are shown the positions occupied by the earth and
its satellite in the midst of the nebulous appendage.
Now, assuming as a positive fact the passage of our planet
through the tail of the comet of 1861, were any special pheno-
mena observed which could be attributed to this singular ren-
contre ? The reply to this question is probably to be found in
the concluding observations of Mr. Hind's letter : 1 1 may add,'
he observes, ' that on Sunday evening, while the comet was so
conspicuous in the northern heavens, there was a peculiar phos-
phorescence or illumination of the sky, which I attributed at
the time to an auroral glare ; it was remarked by other persons
as something unusual, and considering how near we must have
been on that evening to the tail of the comet, it may, perhaps,
be a point worthy of investigation, whether such effect can be
attributed to our proximity thereto. If a similar illumination
of the heavens has been remarked generally on the earth's sur-
face, it will be a significant fact.'
The following note, to a similar effect, appears in the
journal of another English savant, Mr. E. J. Lowe, of High-
field House, near Nottingham : ' June 30 : A singular yellow
phosphorescent glare, very like diffused aurora borealis, yet
being daylight, such aurora would scarcely be noticeable. '*
This is evidently the phenomenon described by Mr. Hind ; but
as both the observations were made in the same country, it
may refer to a merely local appearance.
The fan-like figure presented by the tail of this comet during
the night common to June 30 and July 1, that is to say, at the
precise moment when the passage was taking place is con-
nected, according to M. Liais, with this fact; but in our
opinion the divergence of the rays of the tail might be ex-
[ *Mr. Lowe's letter, containing this extract from his journal, was published
in the Times, July 9, 1861. Mr. Hind's letter appeared in the Times of July 6.
—Bo.]
490
THE TAIL OF THE COMET OF 1801 AND THE EARTH.
plained as a simple effect of perspective. The comet pro-
jecting its tail towards the earth, it is evident that the form of
the appendage, starting from the nucleus, would appear to en-
large considerably, even if its real form were cylindrical and
not conical. On this subject M. Liais writes : ' Those divergent
rays, which lasted so short a time, and were not distinguishable
quite up to the nucleus, might they not be regions of the cir-
cumference rendered visible under the influence of electric
Fig. 75. — Fan-shaped tail of the great comet of 1861 or June 30.
light on leaving the direction of the earth ? To gleams of electric
light shining between the tenuous regions of the tail, and the
limit of our atmosphere, we might with probability attribute
the phosphorescence of the heavens seen on the same evening
by Mr. Hind and other English observers.'
Babinet, in one of his piquant scientific notices, relates,
apropos of the great comet of 1861, the following conversation :
' Monsieur, the newspapers inform us that we have a comet.'
491
THE WORLD OF COMETS.
'Yes, Madame, a very beautiful comet; the history of astronomy
has never recorded one more beautiful.' ' What does it predict ? '
'Nothing at all, Madame.' 'Is it a fine evening?' l Yes, Madame,
splendid, and you have only to go into the garden and you
will see it.' 'Oh ! if it can do one neither good nor harm it is
not worth while.' The lady retires to bed. You will say to me,
' Of what use is astronomy?' ' It is of use,' I reply, ' inasmuch
as we are enabled to go to bed without fear in 1861, even
when a superb comet is in sight. This was not the case six
hundred years ago, or even three hundred.'
Astronomy, as we have seen, has not yet produced this effect
upon every one. But although the earth may have passed
through the tail of a comet without its inhabitants, one or two
excepted, being even conscious of it, still, were our globe
to penetrate to the nucleus of one of these bodies, the event
might not be so harmless. This is a distinction which M.
Babinet, who clings to his theory of visible nonentities, refuses
to make. If, however, the mass of a comet were so small
that its action was imperceptible, it would still remain to
inquire if the introduction of cometary matter into the atmo-
sphere of the earth might not be injurious to living beings.
492
CHAPTER XIV.
PHYSICAL INFLUENCES OF COMETS.
SECTION I.
SUPPOSED PHYSICAL INFLUENCES OF COMF.TS.
The great comet of 1811 ; the comet wire— Prejudices and conjectures— Remark-
able comets and telescopic comets —Comets are continually traversing the heavens.
IN former times when a new comet was seen to project upon
the sky its vaporous star and plume of light, the first question
in the mouth of everyone was, What great calamity does God
announce ?
Even at the present day people may be heard enquiring
what the comet signifies ; 'but the greater number of enquirers
are far more occupied with the physical effects likely to accrue,
than with the supernatural import of the apparition. Do you
think we shall have a warm and dry summer ? is the question
of some. Are we to anticipate foggy weather, heavy rains
and inundations ? ask others. It announces an abundant
harvest, or a superior quality of the year's wine, is gladly
remarked by those who have-not forgotten the comet and the
good wine of the year 1811.
In a word, people readily believe' that the passage of a
comet within sight of the earth must be followed by certain
consequences of a nature to influence not only the climate,
temperature, and vegetation of the latter, but likewise the
health of animals and man, for I have forgotten to say that the
influence of comets upon the production of epidemics and other
495
THE WORLD OF COMETS.
maladies was formerly an article of popular belief. To give an
idea of the prejudices entertained upon this subject not more
than sixty years ago, we will quote from Arago the following
passage taken from the Gentleman's Magazine : —
' Through the influence of the comet of 1811, the
winter following was very mild ; the spring was wet, the
summer cool, and very little appearance of the sun to
ripen the produce of the earth ; yet the harvest was not defi-
cient ; and some fruits not only abundant, but deliciously ripe,
such as figs, melons, and wall fruit. Very few wasps appeared,
and the flies became blind and disappeared early in the season.
... But what is very remarkable, in the metropolis and
about it, was the number of females who produced twins, some
had more, and a shoemaker's wife in Whitechapel produced
four at one birth. ..'* This shows certainly an extravagant
•/ *— '
imagination.
In these entirely conjectural suppositions, especially in
those which are advanced in the form of questions, is there any
base of truth which the astronomical science of the present day
might seem at all to confirm ? In the majority of cases there
is every reason to believe that these assumed influences amount
to nothing ; but then probabilities are not certainties, and a
case might arise in which the apparition of a comet could be
reasonably suspected of having been concerned in the pro-
duction of certain terrestrial phenomena, such as, for example,
meteorological phenomena.
[* This is an extract from a letter which appeared in the Gentleman's
Magazine for November, 1813 (p. 432), and is signed J. B. It has had the
distinction of being quoted by Arago and by M. Giiillemin ; but it should be
remembered that it was merely an anonymous letter, published in an unscien-
tific periodical. It seems to me to be characteristic of a class of letters which all
who are associated with astronomy frequently receive from unscientific people,^
rather than representative of the prejudices prevalent at the time it was written
There are always persons who write letters of this kind, and sometimes, of course,
they ficd their way into print. — ED.]
496
SUPPOSED PHYSICAL INFLUENCE OF COMETS.
Let us examine the principal influences enumerated, and
see if they are confirmed by facts ; and, if not, whether there
is reason to admit, of course with necessary reservations, a
certain amount of probability.
We have already spoken of the influences which comets
must exercise in virtue of their mass. These are indisputable ;
but, up to the present time, as we have seen, the comets of
which history has made mention, and which might have been
expected to produce a disturbing influence, have produced
absolutely no appreciable effect. A comet passing within a
short distance of the earth would in reality act upon the
waters of the sea, and upon the atmosphere, for so short a time
that the wave produced would be insignificant.
But may there not be comets yet unknown of masses more
considerable? May there not be comets which might pass
sufficiently near the earth, and remain long enough in its
vicinity for their masses to occasion an appreciable disturbance?
A shock or a rencontre is improbable, but possible, and, as we
have seen, the consequences that would result from it ar,e mere
conjectures. As for the second question, the different velocities
alone of the earth, and of any comet which might be situated
for a moment in its vicinity, would, as we have already said,
rapidly separate the two bodies. But this is not the kind ot
influence we have here to examine.
In the first place we must remember that comets are more
numerous than might be supposed, that new comets make
their appearance every year, that there are often several in the
course of the year, and that if the influence with which they
are credited belonged to them simply as comets, it would be,
so to speak, continuous. This is no reason for considering it
to be nil, but it is clear that it would be a very difficult matter
to distinguish it from all other causes, regular or irregular.
Those who have admitted, a priori so to speak, the existence
497 K K
THE WORLD OF COMETS.
of such an influence, have scarcely attempted the task of its
verification.
In former times it was to comets visible to the naked eye —
no others were then known — that fatal influences were alone
attributed. And at the present day it is the larger and more
magnificent comets, those which ' make a show ' in the sky,
that are supposed to exert an influence upon our globe. Nor
is this surprising, since the greater or less visibility of a
comet is a measure of its brilliancy and size, or, what comes
nearly to the same thing, of its proximity to the earth.
498
SECTION II.
DO COMETS EXERCISE ANY INFLUENCE UPON THE SEASONS ?
Study of the question by Arago — The calorific action of comets upon the earth
appears to he inappreciable — Comparison of the meteorological statistics of various
years in which comets did and did not appear — The meteorological influence of a
comet is not yet proved by any authentic fact.
WE have already said how general a consternation was created
in 1832 by the announcement that Biela's comet would pass
within a very short distance of the orbit of the earth. Arago
made it the occasion of one of those brilliant and inte-
resting notices in which he endeavoured to destroy existing
prejudices, and to render the simple truths of astronomy better
and more generally understood. The heading of one section
of this notice was —
' Will the future Comet modify in any appreciable degree the
Course of the Seasons of the year 1832 ? '
To this question he replies in the following terms :—
' The above title will doubtless call to mind the beautiful
comet of 1811, the high temperature of that year, the abundant
harvest following, and, above all, the excellent quality of the
comet wine. I am therefore well aware that I shall have to
contend with many prejudices in order to establish that neither
the comet of 1811, nor any other known comet, has ever occa-
sioned the smallest change in the seasons. This opinion is
founded upon a careful examination and attentive discussion
499 K K 2
THE WORLD OF COMETS.
of all the elements of the problem, whilst the opposite idea,
however widely spread it may be, has no foundation whatever
in fact.
4 It is said that comets heat our globe by their presence.
Be this as it may, nothing is easier to verify. Is not the ther-
mometer consulted many times a day in all the observatories
throughout Europe ? Is not an exact record kept of all the
comets which appear ? '
Thereupon Arago proceeds to tabulate the mean tempera-
tures of the years between 1803 and 1831, at the same time
placing by the side of them the numbers of comets observed,
together with any peculiarities exhibited by them which could
exercise an influence upon temperature. He has since extended
this instructive table from 1735 to 1853, and proves without
difficulty that no law connects the variations of mean tempera-
ture with the apparition of comets, and that years fruitful in
apparitions, such as those of 1808, 1819, 1846, for example,
have been marked by temperatures lower, or hardly equal to
those of years in which few or no comets have been seen.
The whole of the sixty -nine comet years give a mean tem-
perature of 51°.46 Fahr. ; twenty-seven years without comets
give a mean of 50°. 94 Fahr. The difference of the half degree
Fahrenheit, Arago explains by the fact that years without
comets are most frequently cloudy ; the prevalence of cloud
simply concealing the comet or comets from observation. This
difference becomes almost inappreciable when he compares the
mean temperatures of the thirty years, in each of which only
one comet appeared, and the thirty-nine years, in each of which
two or several comets appeared. The difference in this case is
no more than four-hundredths of a degree Fahrenheit, a quan-
tity absolutely insignificant.
Other tables, founded upon analogous data, further establish
' that very low temperatures have frequently taken place during
500
DO COMETS EXERCISE ANY INFLUENCE UPON THE SEASONS?
the apparition of comets, and very high temperatures at epochs
when none of these bodies have been visible.'
Returning, then, to the comet of 1811, Arago considers
how far it was possible for the brilliant train of that body to
exercise an influence upon our globe ? It was 102,000,000 of
miles in length, it is true ; but then it was not strictly directed
towards the earth, and the comet at its least distance from
our globe was separated by 1 1 7,000,000 of miles. Moreover,
we are now assured of the extreme tenuity of these cometary
appendages, and the in significant amount of heat which they
have it in their power to communicate, either at a distance by
means of reflexion, or by contact. But the result might be
different in the event of contact with the nucleus if, as is pro-
bably the case, the matter of which the nucleus is composed
should have become heated, in the neighbourhood of the sun,
to so high a temperature as to cause its partial incandescence.
Arago's demonstration did not succeed in convincing every
one, for after the apparition of Halley's famous comet, the mild
temperatures of the months of October and November were
ascribed by many persons to the passage of the comet. ' People
wish,' he observes, ' to attribute the mild temperature enjoyed
by the north of France during these eight weeks to the influ-
ence of the comet ! I could,' he continues, ' instance on the
one hand Octobers and Novembers still milder than those of
1835, when no comets were visible, and on the other I could
find instances of great cold being experienced during the
same months, when brilliant comets were in sight ; but to
come more directly to the point, I will remark that at the
end of 1835, when Paris was enjoying a very mild tem-
perature, it was especially cold in the south, so that if the
temperature were dependent upon the comet, its action would
have to vary with the position of the place.'
And further, in order to judge the question fairly by this
501
THE WORLD OF COMETS.
method, that is to say, by the comparison of meteorological
statistics, it is clear that we must not be content with observa-
tions relating only to one region of the earth. In order to
form an impartial judgment, we must decide whether the pre-
sence or proximity of the comet corresponds to an increase of
temperature over the whole of the terrestrial globe, or at least
over all that portion of the globe which occupies the same
relative position with regard to the comet.
The comet now in sight [July 1874] is observed by the
public at the hottest time of the year, and it is probable that,
without seeking further for a cause, many people attribute to
the comet the high temperature from which they suffer. This
present year may be in France and even throughout Europe
a warm year. But is it so too for the same latitudes in
America? Coggia's comet is the third of the year 1874; but
we must not forget that in 1873 no less than seven comets
passed their perihelia.
In conclusion, we may say that the influence of a comet
upon the temperature and the seasons is generally impercep-
tible. It could only become sensible on the hypothesis of a
collision, or a very near approach between the earth and a
comet. Finally, up to the present time we have no authentic
instance of such an influence. Mere opinions which are
not justified by examination of the facts are but valueless
hypotheses.
502
SECTION III.
PENETRATION OF COMETARY MATTER INTO THE TERRESTRIAL
ATMOSPHERE,
Is this penetration physically possible ? — Cometary influences, according to Dr. Forster
—Were the dry fogs of 1783, 1831, and 1834, due to the tails of comets ?— Volcanic
phenomena and turning turf-beds; their probable coincidence with fogs — Pro-
bable hypothesis of Franklin — Dry fogs, atmospheric du»t, and bolides.
WE perceive, then, that the influence of comets upon living
beings by the action of heat is a hypothesis which, for the
present, must be abandoned ; in so far, at least, as the action
of heat by radiation from a distance is concerned. We have
throughout reserved the questions of a collision between the
two bodies, and of the penetration of the earth to the heart of
a mass in a state of incandescence.
Apart from the action of calorific radiation, what influence
of any other kind could a comet exercise upon the meterolo-
gical conditions of the earth ? We know of absolutely none.
It remains, then, to consider the immediate physical or
chemical influence of the cometary substance. It is not for-
bidden to our globe, as we have seen, to traverse the gigantic
trains which form the tails of certain comets, nor to penetrate
to a certain depth the vaporous atmosphere of some amongst
them. Apart from these rencontres, we may suppose that
cometary matter may be introduced into our atmosphere by
the power of attraction. Pursuing its course in the same
503
THE WORLD OF COMETS.
regions as the planets, projecting its substance far beyond
its own sphere of attraction, a comet can scarcely fail to
abandon fragments of its tail, which the mass of the earth,
for example, may afterwards appropriate.
These fragments, it is true, by the common consent of all
astronomers, are but trifling, materially speaking, and their
total weight is only a very insignificant fraction of that of our
atmosphere ; but might not the continued introduction of these
particles into the air we breathe become in the course of time
a source of sickness and death to the living beings inhaling
them? Might not certain kinds of epidemics be thus explained?
This is a question which we scarcely have the means of answer-
ing. If the tails of comets are formed of matter so attenu-
ated, so little coherent, it is reasonable to suppose that they may
be attracted to the earth and become an integral part of it.
But how are we to suppose that they descend into the depths of
this envelope? At the utmost they could only float at the ex-
treme limit of the atmosphere, and the supposed gas of which
they are composed would not in any way mix with the gases of
the air which human beings and animals respire.* Suppose
these gaseous particles are endowed with a peculiar chemical
activity, and that their contact with oxygen or nitrogen deter-
mines the formation of dense and poisonous precipitates : even
then these particles of a matter so prodigiously dilated in the be-
ginning, would contribute when condensed but an infinitesimal
quantity to the air we breathe, and, unless we have faith in the
homoeopathic doctrine, need inspire us with no alarm.
Appeal has been made to the facts. As writers who believed
in the supernatural and providential influence of comets have
[* If we suppose the tail of the comet to consist of a gas, it would mix with
the other gases of the atmosphere in accordance with the known law of diffusion.
If a light gas be placed upon a heavy gas, the latter will not remain floating as it
were upon the former, but after a time the two will become completely mixed.
—En.]
504
PENETRATION OF COMETARY MATTER INTO THE ATMOSI'HKIIK.
collected all historical details attending upon each apparition
which might seem to bear testimony in favour of their
superstition, so have the advocates of a connexion between epi-
demics and comets collected a number of supposed accord-
ances. Arago quotes from Dr. T. Forster, who had expended
a large amount of erudition in forming a catalogue of so-called
cometary influences.
4 Mr. Forster has,' he observes, ' so extended the circle of
supposed cometary actions that there is scarcely a phenomenon
in nature that may not be ascribed to cometary influence.
Cold and warm seasons, earthquakes, volcanic eruptions, great
hail-storms, abundant snows, heavy rains, floods, droughts,
famines, thick clouds of flies or locusts, the plague, dysentery,
epizootic diseases, all are recorded by Mr. Forster with refer-
ence to each cometary apparition, no matter what the continent,
kingdom, town, or village subjected to the ravages of the plague,
famine, &c.' For example, the date of the comet of 1668
corresponds to the remark that all the cats in Westphalia were
sick; that of 1746 to the earthquake in Peru which destroyed
Lima and Callao ; other dates, again, correspond to the fall of
an aerolite, to the passage of numerous flocks of pigeons, &c.
This is truly an absurd enumeration. It recalls to mind
Bayle's letter, and his parallel of the lady and the carriages in
the Rue St. Honore. But the whole is too puerile to need
refutation.
Of meteorological phenomena attributed to comets because
their causes have remained unknown, mention must be made
of dry fogs, such as those of 1783, 1822, 1831, and 1834.
The appearance of this singular phenomenon, and the
circumstances which, in 1783 more particularly, accompanied
its long duration (it was visible more than a month), explain
to a certain extent this hypothesis. Hygrometrically, this
fog had not the qualities of an ordinary fog: it was not wetting.
605
THE WORLD OF COMETS.
De Saussure's hygrometer marked only 57°; the general colour
of the air was that of a dull, dirty blue; distant objects were
blue, or surrounded by mist, and at the distance of a league
were undistinguishable. The sun, red, without brilliancy, and
obscured by mist, both at his rising and setting, could be
steadfastly regarded at noonday. A singular circumstance
mentioned by Arago is that the dry fog of 1783 appeared to
possess a certain phosphoric property, a light of its own. ' I
find at least in the accounts of some observers,' he remarks,
* that it diffused, even at midnight, a light which they compare
to that of the moon at its full, and which sufficed to make
objects distinctly visible at a distance of more than 200 yards.'
Was the earth plunged in the tail of a comet, or had it met
with the fragment of a cometary appendage abandoned in
space ? But why, then, was not the comet itself visible ?
Meteorologists (Kamtz) still continue to rank dry fogs amongst
problematical phenomena ; nevertheless, it was remarked that
in 1783. at the two extremities of Europe, violent physical
commotions took place ; continued earthquakes in Calabria,
and a volcanic eruption in Iceland. Could the dust and ashes
projected to a distance and scattered far and wide have been
the cause of the phenomenon ?
Dry fogs are common in Holland, and also in the west and
north of Germany. Finke tells us that they are due to the
smoke produced by the combustion of the turf-beds. In 1834
the drought did in fact cause numerous fires in the forests and
turf-beds of Prussia, Silesia, Sweden, and Russia.
Franklin assumed, in order to explain the dry fog of 1783,
the diffusion of volcanic cinders and emanations. He likewise
supposes — and this hypothesis is closely allied to that of the
earth's immersion in the train of a comet — that an immense
bolide might penetrate into our atmosphere, be there im-
perfectly consumed, and diffuse torrents of smoke or light
506
PENETRATION OF COMET ARY MATTER INTO THE ATMOSPHERE.
ashes. We shall presently see that certain rains of dust can
be explained in a similar manner. Many savants admit as a
very probable fact that matter of extra-terrestrial origin may
penetrate into the atmosphere and fall to the ground, and per-
haps modify the constitution of the gaseous envelope in which
we live.
SECTION IV.
CHEMICAL INFLUENCES OF COMETS.
Introduction of poisonous vapours into the terrestrial atmosphere — The end of the
world and the imaginary comet of Edgar Poe ; Conversation of Eiros and Charmion
— Poetry and Science ; impossibilities and contradictions.
WE now come to that other cometary influence which we have
already alluded to, an influence capable of changing the air
we breathe by the introduction of foreign effluvia.
Nothing within the range of fact and observation, up to
the present time, affords ground for belief in such an influence.
But this hypothesis has had the fortune to be presented in a
striking and practical form by a modern writer of powerful
imagination. The American poet Edgar Poe, whose Extraor-
dinary Histories are known to everyone, has placed in the
mouth of a being who has suffered death, an account of the
destruction of the world by the near approach of a comet.
We subjoin the principal portion of this wonderful dream,
in which Eiros relates to Charmion the circumstances which
put an end to the world.
' The individual calamity was, as you say, entirely unan-
ticipated, but analogous misfortunes had been long a subject
of discussion with astronomers. I need scarce tell you, my
friend, that, even when you left us, men had agreed to under-
stand those passages in the most holy writings which speak of
o08
CHEMICAL INFLUENCES OF COMETS.
the final destruction of all things by fire as having reference
to the orb of the earth alone. But in regard to the immediate
agency of the ruin, speculation had been at fault from th»t
epoch in astronomical knowledge in which the comets were
divested of the terrors of flame. The very moderate density
of these bodies had been well established. They had been
observed to pass among the satellites of Jupiter without bring-
ing about any sensible alteration either in the masses or in
the orbits of these secondary planets.
' We had long regarded the wanderers as vapoury creations
of inconceivable tenuity and as altogether incapable of doing
injury to our substantial globe, even in the event of contact.
But contact was not in any degree dreaded, for the elements
of all the comets were accurately known. That among them
we should look for the agency of the threatened fiery destruc-
tion had been for many years an inadmissible idea. But
wonders and wild fancies had been, of late days, strangely rife
among mankind ; and, although it was only with a few of the
ignorant that actual apprehension prevailed upon the an-
nouncement by astronomers of a new comet, yet this announce-
ment was generally received with I know not what of agitation
and mistrust.
' The elements of the strange orb were immediately calcu-
lated, arid it was at once conceded by all observers that its
path, at perihelion, would bring it into very close proximity
with the earth. There were two or three astronomers of
secondary note, who resolutely maintained that a contact was
inevitable. I cannot very well express to you the effect of this
intelligence upon the people. For a few short days they would
not believe an assertion which their intellect, so long employed
among worldly considerations, could not in any manner grasp.
But the truth of a vitally important fact soon made its way
into the understanding of even the most stolid. Finally, all
609
THE WORLD OF COMETS.
men saw that astronomical knowledge lied not, and they
awaited the comet. Its approach was not at first seemingly
rapid ; nor was its appearance of very unusual character.
It was of a dull red, and had little perceptible train. For
seven or eight days we saw no material increase in its apparent
diameter, and but a partial alteration in its colour. Meantime,
the ordinary affairs of men were discarded, and all interest
absorbed in a growing discussion, instituted by the philosophic,
in respect to the cometary nature. Even the grossly ignorant
aroused their sluggish capacities to such considerations. The
learned now gave their intellect — their soul — to no such points
as the allaying of fear, or to the sustenance of loved theory.
They sought — they panted for right views. They groaned for
perfected knowledge. Truth arose in the purity of her strength
and exceeding majesty, and the wise bowed down and adored.
' That material injury to our globe or to its inhabitants
would result from the apprehended contact, was an opinion
which hourly lost ground among the wise, and the wise were
now fully permitted to rule the reason and the fancy of the
crowd. It was demonstrated that the density of the comet's
nucleus was far less than that of our rarest gas ; and the harm-
less passage of a similar visitor among the satellites of Jupiter
was a point strongly insisted upon, and which served greatly
to allay terror. Theologists, with an earnestness fear-enkind-
led, dwelt upon the Biblical prophecies, and expounded them to
the people with a directness and simplicity of which no previous
instance had been known. That the final destruction of the
earth must be brought about by the agency of fire, was urged
with a spirit that enforced everywhere conviction ; and that
the comets were of no fiery nature (as all men now knew) was
a truth which relieved all, in a great measure, from the ap-
prehension of the great calamity foretold. It is noticeable that
the popular prejudices and vulgar errors in regard to pestilence
510
CHEMICAL INFLUENCES OF COMETS.
and wars — errors which were wont to prevail upon every ap-
pearance of a comet — were now altogether unknown. As if by
some sudden convulsive exertion reason had at once hurled
superstition from her throne. The feeblest intellect had
derived vigour from excessive interest. What minor evils
might arise from the contact were points of elaborate ques-
tion. The learned spoke of slight geological disturbances,
of probable alterations in climate, and consequently in vegeta-
tion ; of possible magnetic and electric influences. Many held
that no visible or perceptible effect would in any manner be
produced. While such discussions were going on, the subject
gradually approached, growing larger in apparent diameter,
and of a more brilliant lustre. Mankind grew pale as it
came. All human operations were suspended.
1 There was an epoch in the course of the general sentiment
when the comet had attained, at length, a size surpassing that
of any previously recorded visitation. The people now, dis-
missing any lingering hope that the astronomers were wrong,
experienced all the certainty of evil. The chimerical aspect of
their terror was gone. The hearts of the stoutest of our race
beat violently within their bosoms. A very few days sufficed,
however, to merge even such feelings in sentiments more un-
endurable. We could no longer apply to the strange orb any
accustomed thoughts. Its historical attributes had disappeared.
It oppressed us with an hideous novelty of emotion. We saw
it not as an astronomical phenomenon in the heavens, but as
an incubus upon our hearts, and a shadow upon our brains.
It had taken, with inconceivable rapidity, the character of a
gigantic mantle of rare flame, extending from horizon to
horizon.
' Yet a day, and men breathed with greater freedom. It
was clear that we were already within the influence of the
comet; yet we lived. We even felt an unusual elasticity of
511
THE WORLD OF COMETS.
frame and vivacity of mind. The exceeding tenuity of the
object of our dread was apparent, for all heavenly objects were
plainly visible through it. Meantime, our vegetation had per-
ceptibly altered ; and we gained faith, from this predicted
circumstance, in the foresight of the wise. A wild luxuriance
of foliage, utterly unknown before, burst out upon every veget-
able thing.
' Yet another day — and the evil was not altogether upon us.
It was now evident that its nucleus would first reach us. A
wild change had come over all men ; and the first sense of
pain was the wild signal for general lamentation and horror.
This first sense of pain lay in a rigorous constriction of the
breast and lungs, and an insufferable dryness of the skin. It
could not be denied that our atmosphere was radically affected ;
the conformation of this atmosphere, and the possible modifica-
tions to which it might be subjected, were now the topics of
discussion. The result of investigation sent an electric thrill
of the intensest terror through the universal heart of man.
' It had been long known that the air which encircled us was
a compound of oxygen and nitrogen gases, in the proportion of
twenty-one measures of oxygen, and seventy-nine of nitrogen,
in every one hundred of the atmosphere. Oxygen, which was
the principle of combustion and the vehicle of heat, was ab-
solutely necessary to the support of animal life, and was the
most powerful and energetic agent in nature. Nitrogen, on
the contrary, was incapable of supporting either animal life or
flame. An unnatural excess of oxygen wTould result, it had
been ascertained, in just such an elevation of the animal spirits
as we had latterly experienced. It was the pursuit, the ex-
tension of the idea, which had engendered awe. What would
be the result of a total extraction of the nitrogen ? A combustion
irresistible, all devouring, omni-prevalent, immediate ; — the
entire fulfilment, in all their minute and terrible details, of the
512
CHEMICAL INFLUENCES OF COMETS.
fiery and horror-inspiring denunciations of the prophecies of
the Holy Book.
' Why need I paint, Charmion, the now disenchained frenzy
of mankind ? That tenuity in the comet which had previously
inspired us with hope, was now the source of the bitterness of
despair. In its impalpable gaseous character we clearly per-
ceived the consummation of Fate. Meantime a day again
passed — bearing away with it the last shadow of Hope. We
gasped in the rapid modification of the air. The red blood
bounded tumultuously through its strict channels. A furious
delirium possessed all men ; and with arms rigidly outstretched
towards the threatening heavens, they trembled and shrieked
aloud. But the nucleus of the destroyer was now upon us : —
even here in Aidenn. I shudder while I speak. Let me be
brief — brief as the ruin that overwhelmed. For a moment there
was a wild lurid light alone, visiting and penetrating all things.
Then — let us bow down, Charmion, before the excessive
majesty of the great God ! — then there came a shouting and
pervading sound as if from the mouth itself of HIM, while the
whole incumbent mass of ether in which we existed burst at once
into a species of intense flame, for whose surpassing brilliancy
and all-fervid heat even the angels in the high Heaven of pure
knowledge have no name. Thus ended all.' *
Like most of the productions of the American poet the frag-
ment we have quoted bears the stamp of a well marked origin-
ality. It is a curious blending of the conceptions of the poet
with the philosophical descriptions and the positive, realistic
analyses of the savant. This much to be desired employment
of science in poetry and art is characteristic of the talent or
rather, perhaps, the genius of Edgar Poe, and has the effect of
producing an intense and keen emotion in the mind of the
reader.
1 « The Conversation of Eiros and Charmion.'— Poe's Works, vol. ii.
613 L L
THE WORLD OF COMETS.
Unfortunately the savant has not been equal to the poet.
And we cannot read his assertions respecting cometary astro-
nomy without smiling at the inaccuracies and even blunders
into which the author has fallen. It is a great defect, since
the emotion which he designs to inspire misses its effect, as
soon as the reader perceives the want of accord between the
fact and the dream. But on the other hand we see that Poe
has designedly neglected to employ any of the ordinary catas-
trophes which have been supposed likely to result from the
rencontre of a comet and the earth. He calls in aid neither
flood nor fire, in the ordinary sense, nor the disruption of the
earth. Not even poison, nor the respiration of a poisonous
matter. A simple addition, in increasing proportion of oxygen
gas, and all is told. It is true that he speaks, we know not
why, of a total extraction of nitrogen ; we seek in vain for the
scientific reason for this extraction. Nor is the final piercing
sound and the explosion easier to understand. The effects are
not as described, when a living creature is subjected to an in-
creased pressure of an oxygenated atmosphere, as M. Bert's ex-
periments have shown. But a final coup de theatre was needed,
and on this point Poe has made a sacrifice to the vulgar.
Other observations might be made ; but we have already
explained the nature of the laws applying to cometary move-
ments, and the reader will not fail to detect the errors of the
poet, who, were he writing at the present day, would be obliged
to change the form and manner of his catastrophe. The known
results of spectral analysis would no longer permit him to
represent a comet as an agglomeration of oxygen. Likewise
nothing proves that the matter of which a comet is composed
is in a gaseous state ; the nucleus on the contrary would appear
to be either a solid or a liquid mass, and the atmosphere with
which it is surrounded on all sides an aggregation of isolated
particles.
514
CHAPTER XV.
SOME QUESTIONS ABOUT COMETS,
615 , t L 2
SECTION I.
ARE COMETS HABITABLE?
The inhabitants of comets as depicted in the Plurality des Mondes of Fontenelle —
Ideas of Lambert respecting the habitability of comets — That comets are the abode
of human beings is a hypothesis incompatible with the received facts of astronomy.
AFTER NEWTON, and especially in the eighteenth century, by a
not unnatural reaction of ideas from the Aristotelian doctrine of
transient meteors, comets were regarded as bodies, stable and
permanent as the planets ; they were obedient to the same laws
of movement, and differed only as regards appearance, by their
nebulosities and tails. The astronomers of that time, taken
up with the verification and calculation of their positions and
orbits, occupied themselves little or not at all with the study
of details which were purely physical, such as are now called
cometary phenomena. Regarding them as spheroids, solid like
the planets, and similar to them in the constituents of their
nuclei, to people them with inhabitants followed in the
natural sequence of ideas.
Fontenelle, who, as we know, was a believer in the theory
of vortices, and who, moreover, regarded the heads and tails of
comets as simple optical appearances, thus expresses himself in
the Pluralite des Mondes.
1 Comets,' he observes, ' are planets which belong to a
neighbouring vortex ; they move near the boundaries of it ; but
this vortex, being unequally pressed upon by those that are
517
THE WORLD OF COMETS.
adjacent to it, is rounder above and flatter below, and it is the
part below that concerns us. Those planets which near the
summit began to move in circles did not foresee that, down
below, the vortex would fail them, because it is there as
it were crushed. Our comet is thus forced to enter the
neighbouring vortex, and this it cannot do without a shock.'
Also further on, Fontenelle observes, returning to the same
point : ' I have already told you of the shock which takes
place when two vortices meet and repel each other. I believe
that in this case the poor comet is rudely enough shaken and its
inhabitants not less so. We deem ourselves very unfortunate
when a comet appears in sight ; but it is the comet itself which
is very unfortunate.' 'I do not think so,' said the Marquise ; 'it
brings to us all its inhabitants in good health. Nothing is so
delightful as thus to change vortices. We who never quit ours
lead a life wearisome enough. If the inhabitants have sufficient
knowledge to predict the time of their entrance into our world,
those who have already made the voyage announce beforehand
to others what they will see.' ' You will soon discover a
planet which has a great ring about it, they will say perhaps,'
speaking of Saturn. * You will see another which will be
followed by four little ones. Perhaps even there are people
appointed to look out for new worlds as they appear in sight,
and who cry immediately, A new sun ! a new sun ! as sailors
cry, Land ! land ! Believe me, we have no need to pity the
inhabitants of a comet.'
Lambert in his Lettres Cosmologiques (1765) devotes a
chapter to the question, Are comets habitable ? Guided by con-
siderations foreign to science, and dominated by a preconceived
idea that all globes must be inhabited, he seeks to discover
reasons which may permit us to believe that comets, more
numerous than the planets in the solar system, are habitable
celestial bodies.
518
ARE COMETS HABITABLE ?
A first difficulty arises from the extremes of temperature to
which cornets are subjected at their aphelia arid perihelia.
' How are we to conceive,' he observes, * that beings can exist
in an abode which is subjected to the utmost extremes of
heat and cold ? The comet which appeared in 1759 (that of
Halley) and which returns the quickest of all those whose
periods are known undergoes a winter 70 years long. But
there is even a greater extreme of heat.' Although Lambert
objects to Newton's calculation as to the heat to which the
comet of 1680 must have been subjected during its perihelion
passage, still he is obliged to admit that on the 8th December,
1680, 'the comet being one hundred and sixty times nearer to
the sun than we are ourselves, must have been subjected to a
degree of heat tv\7enty-five thousand six hundred times as
great as we are. Whether this comet was of a more compact
substance than our globe, or was protected in some other way,
it made its perihelion passage in safety, and we may suppose
all its inhabitants also passed safely. No doubt they would
have to be of a more vigorous temperament and of a constitu-
tion very different from our own. But why should all living
beings necessarily be constituted like ourselves ? Is it not
infinitely more probable that amongst the different globes of
the universe a variety of organizations exist, adapted to the
wants of the people who inhabit them, and fitting them for the
places in which they dwell, and the temperatures to which they
will be subjected? Have we not in like manner abandoned the
prejudice which for a length of time caused the torrid and
frigid zones to be regarded as uninhabitable? Is man the only
inhabitant of the earth itself ? And if we had never seen
either bird or fish, should we not believe that the air and water
were uninhabitable? Are we sure that fire has not its invisible
inhabitants, whose bodies, made of asbestos, are impenetrable
to flame? Let us admit that the nature of the beings who
519
THE WORLD OF COMETS.
inhabit comets is unknown to us ; but let us not deny their
existence, and still less the possibility of it.'
Thus regarded as a matter of pure hypothesis, it is plain
that the question of the habitability of comets may always be
answered in the affirmative. But we must not forget that at
the time when Lambert wrote, comets were regarded as solid
bodies enveloped by a considerable atmosphere, and the ten-
dency to assimilate them to the planets was general ; add to
this a few vague ideas upon the subject of final causes — such as
Lambert held — and it was natural to people all the stars of
heaven, and even the sun himself with inhabitants.
Andrew Oliver published nearly about the same date
(1772) an Essay upon Comets, wherein he seeks to ex-
plain the formation of tails by a mutual repulsion of electric
origin, between the atmospheres of the sun and the comet:
he devotes the second part of his curious work to showing
that the tails of comets are probably intended to render
their bodies habitable worlds. The enormous variations of
temperature to which a comet is subjected in passing from one
extremity of its orbit to the other, are exactly or at least suit-
ably compensated by variations in the density of its atmosphere.
This, together with the movements due to the action of the sun
and the supposed velocity of rotation, prevents the extremes of
heat and cold from becoming intolerable. At the aphelion
both its atmosphere and tail are condensed about the comet,
and the air is in a state of perfect calm. In proportion as it
approaches perihelion, the atmosphere becomes rarified, the
equilibrium is constantly broken, and currents of fresh air temper
the> extreme ardour of the solar rays.
These, as we see, are but physical romances composed by
the partisans of a preconceived idea of the habitability of these
bodies. Neither Fontenelle, nor Lambert, nor Andrew Oliver
would probably write at the present day as they did a hundred
520
ARE COMETS HABITABLE ?
or a hundred and fifty years ago. And for this two reasons
may be assigned, the one philosophical, the other scientific. In
the first place the a priori has by common consent been
banished from science, which leaves to metaphysics the task of
supporting theses by arguments based upon ideas such as that of
final causes. We no longer ask for example how comets must
be constituted to permit the existence of living beings, which
Providence could not have withheld from bodies so numerous
and important. But we seek by the study of observed facts
and by the discussion of the probable physical consequences
which must follow from these facts to form an approximate idea
of the conditions — physical, luminous, calorific, and chemical —
of known comets. And should he then enter upon the question
of the habitability of these bodies, we do not consider it in the
absolute and unconditional manner in which it was entertained
by Lambert. We merely compare the probable conditions as
determined by observation with those which seem to be com-
patible on the surface of the terrestrial globe with the existence
of organized living beings. In short, there has been a total
change of method.
A second reason which would have brought about a change
in the opinions of the eminent savants whose theories we have
just quoted, is that within the last hundred years — as we have
seen in detail — the physical and even chemical constitution of
comets has been carefully studied. We no longer assimilate them
to the planets except as regards their movement of translation.
Everything leads us to believe that the agglomerations of which
they are composed are in a rudimentary state analogous to
the rudis indigestaque moles of chaos. The incessant trans-
formations which take place in their nuclei, their atmospheres
and their tails indicate aii equilibrium eminently unstable, and
which would be very difficult to reconcile with the known con-
ditions of life.
521
THE WORLD OF COMETS.
After this, let all who please picture to themselves the
comet which has lately paid us so brief a visit [July, 1874],
peopled with astronomers such as those of whom Lambert*
speaks. We will not cavil Avith them; we do not fight with
shadows.
* ' I like to picture to myself,' he sayp, ' these globes, voyaging in space, and
peopled with astronomers, who are there on purpose to contemplate nature on a
grand scale, as we contemplate it on a small scale. From their moving observatory,
as it is wafted from sun to sun, they see all things pass successively before their
view, and can determine the positions and motions of all the stars, measure the
orbits of the comets and the planets which glide by them, see how the particular
laws develop into general laws, and know, in a word "the details of the universe."
In truth, I picture to myself that astronomy must be for the inhabitants of such
a comet a terribly complicated science. But doubtless their intelligence is pro-
portional to the difficulties.'
SECTION II.
WHAT WOULD BECOME OF THE EARTH IF A COMET WERE TO
MAKE IT ITS SATELLITE ?
Conditions of temperature to which the earth would be subjected if it were compelled
by a comet to describe the same orbit as the latter — The comets of Halley, and of
1680, examined from this point of view — Extremes of heat and cold : opinion
of Arago : impossibility of living beings resisting such changes.
AEAGO has examined, in an indirect manner, the question of
the habitability of comets; that is to say, he has considered
how far the enormous distances through which a body passes
in describing a very eccentric orbit around the sun, such as
that of a cometary orbit, are compatible with the existence of
inhabitants similar to man. Could the earth, he enquires, ever
become the satellite of a comet, and, if so, what would be the
fate of its inhabitants?
Arago, basing his reasoning upon the comparative smallness
of the masses of comets, regards the transformation of the
earth into the satellite of a comet, as an event ' within the
bounds of possibility, but which is very improbable,' an
opinion no one at the present day will be inclined to dispute.
He next supposes our earth successively made tributary to the
comet of Halley and to that of 1680, and proceeds to con-
sider the conditions of temperature to which our globe would
be subjected whilst travelling in company with each.
523
THE WORLD OF COMETS.
With the comet of Halley our year would be sixty-five
times longer than at present. ' In this period of seventy-
five years, which the new year of the earth would include, five
would be expended in passing over that portion of the curve
comprised within the orbit of Saturn. Let us regard these
five years as equivalent to the summer and the temperate
seasons; there would still remain seventy (the number given
by Lambert), which would belong entirely to winter. At the
time of the comet's perihelion passage, the earth, the satellite
of the comet, would receive from the sun a quantity of rays
three times greater than that which it now receives. At its
aphelion, thirty-eight years after, the quantity of rays received
would be twelve hundred times less than now.'
With the great comet of 1680, the year would be equal to
575 of our years, if we assume Whiston's period. The distance
of the earth from the sun would vary, during this long period,
from the y^^th part of its actual mean distance, to more than
138 times this distance. Our least distance from the sun would
thus be to our greatest distance in the proportion of 6 to
138,296, or of I to 23,050. The intensity of the heat received
at the perihelion would be 28,000 times as great as the actual
mean heat. What would be the effect of such a condition of
things? We can no longer admit with Newton that the heat
acquired by the comet was 2,000 times that of red-hot iron,
when on December 17, 1680, it passed within so small a dis-
tance of the sun. ' This last result,' says Arago, ' was founded
upon incorrect data. The problem was much more com-
plicated than was supposed by Newton, or could have been be-
lieved at the time when the Principia was published. It is
now known that in order to find the temperature which a
determinate quantity of heat could communicate to a planetary
body it is necessary to know the state of the surface of that
body and of its atmosphere.' On these points nothing is known
524
THE EARTH A SATELLITE TO A COMET.
as regards the comet of 1680, but the case is different with the
earth. Arago, however, merely confines himself to the
following remarks :
4 There is no doubt that at first the solid envelope of the
earth would experience a degree of heat 28,000 times greater
than that of summer ; but soon the seas would turn into
vapour, and the thick beds of clouds arising therefrom would
protect it from the conflagration, which at first would seem to
be inevitable. It is therefore certain that the vicinity of
the sun would cause a great increase of temperature, the nu-
merical value of which, from the nature of things, we should be
unable to assign.'
At the aphelion, the distance being 138 times the actual
distance, the heat received from the sun by the earth would be
about 19,000 times less than the mean heat at present. ' Concen-
trated in the focus of the largest lens,' Arago justly observes,
1 it would certainly produce no sensible effect even upon an
air thermometer. The temperature of our globe would then
depend only upon the heat which might remain undissipated
of that which it had received during its perihelion passage,
and upon the intrinsic heat of the regions of space in the
neighbourhood of the aphelion.'
To take the worst possible case, he assumes the loss to be
complete, and the heat of the perihelion to be entirely dissi-
pated. The extreme degree of cold would then be hardly
more than — 58° Fahrenheit. Reasoning upon the fact that voy-
agers to the Polar regions, Franklin for example, in 1820, have
endured a cold of — 57°'5 * Fahrenheit, and likewise upon
* [In the Polar Expedition that has just returned the Alert experienced a
temperature of -73°'7' Fahr., and the mean temperature for thirteen days was
-58°'9', and for five days and nine hours -66°'29'. Although I mention these
facts, I need scarcely say that I do not share Arago's opinion that it would be
possible for human beings to support the extreme temperatures that would result
if the earth were to become a satellite of the comet of 1680.— ED.]
625
THE WORLD OF COMETS.
experiments, which have shown that man can, under special con-
ditions, support a heat of 266° Fahrenheit, he arrives at the
unexpected conclusion ' that there is nothing to prove that if
the earth had become a satellite of the comet of 1680, the human
race would have disappeared through the effects of temperature.'
Let us in the first place consider only the extreme
values — viz. the quantity of heat received by our globe at its
perihelion and aphelion, independently of the physiological
effects. At the perihelion, the amount of heat would be equal
to 28,000 times the actual heat that we now experience; at the
aphelion it would be 19,000 times less. Thus in the first case
it would then be 532 millions of times greater than in the
second.
What human organization could bear these inconceivable
extremes of heat and cold ? How could Arago believe, taking
into account only the immediate action of a temperature so
high, and of a cold so intense, that our constitution would not
be infallibly destroyed between them? But if we ask ourselves
what on such an hypothesis would become of our globe
itself, its land and sea, its climate, vegetation, &c., could the
answer be for a moment doubtful? As regards vegetation
o o
alone, do we not see that a few degrees, more or less, a little
dryness or humidity in excess or defect, causes death to it, or
renders it unproductive? Corn does not yield grain in the
tropics, and man, although by his industry and by artificial
means he is enabled to endure climates to which he is un-
accustomed, with all the precautions in the world does not be-
come acclimatized when he passes from one zone to the other.
Now all that lives on the surface of the earth is consti-
tuted to exist under certain conditions of day and night, and
alternations of the seasons, in an atmosphere whose chemical
composition, density and hygrometrical conditions remain con-
stant or vary only within very restricted limits. What sub-
526
THE EARTH A SATELLITE TO A COMET.
versions, what revolutions in the habits and conditions most
indispensable to existence must there be, if the earth were com-
pelled to follow the movements of a comet, such as that of
1680 ! To a certainty, one revolution of the comet would see
the annihilation of the human race, together with the greater
part of the fauna and flora of the world. Naturalists of the
present day have every reason to believe that the trans-
formations revealed by palaeontological study have been pro-
duced in different ages by corresponding modifications, whether
slow or sudden it matters little, in the physical state of the
atmosphere and earth. Nevertheless, to explain these changes,
there is no need to attribute to the modifications in question
an extent at all comparable to those which would result from
the transformation of the earth into the satellite of a comet
experiencing a range of temperature varying from 28,000
times the actual heat we habitually experience to a quantity
19,000 times less.
Happily for our globe, as Arago has himself admitted, the
probability of an event of this kind is so extremely small,
that we may dismiss it from our minds. It is one of those
problems of pure curiosity, the examination of which merely
furnishes the mind with terms of comparison between what is
and what might be, between what is around us in the world
that we inhabit, and what may be in worlds different to
ours.
527
SECTION III.
IS THE MOON AN ANCIENT COMET ?
Hypothesis of Maupertuis : the planetary satellites originally comets, which have been
retained by the attractions of the planets — The Arcadians and the moon — Refutation
of this hypothesis by Dionys du Sejour.
IN the same spirit of speculative enquiry, it has likewise been
asked if the moon is not an ancient comet which the earth has
diverted from its orbit about the sun, and forced to gravitate
about itself. ' Kot only,' says Maupertuis, 'might a comet
carry away our moon, but it might itself become our satellite,
and be condemned to perform its revolutions about our earth
and illuminate our nights. Our moon might have been
originally a small comet which, in consequence of having too
nearly approached the earth, has been made captive by it.
Jupiter and Saturn, bodies much larger than the earth, and
whose power extends to a greater distance, and over larger
comets, would be more liable than the earth to make such acqui-
sitions ; consequently Jupiter has four moons revolving about
him. and Saturn five.'
Upon what foundation, upon what serious reasoning has
Maupertuis erected this ingenious hypothesis ? He does not tell
us. Pingre, who records it, observes that the partisans of this
opinion based it upon an ancient tradition mentioned by Ovid
and Lucian. The Arcadians were persuaded that their ances-
528
IS TI1E MOON AN ANCIENT COMET ?
tors inhabited Arcadia before the moon existed. But such a
belief furnishes a very poor argument. The arguments, or
rather the calculations, by which Dionys du Sejour has anni-
hilated the hypothesis of Maupertuis are more difficult to
refute. I will briefly state them as given by Pingre* : ' Sub-
jected to the test of analytical reasoning,' he observes, ' the whole
theory falls to the ground. Dionys du Sejour has proved :
1st. That it is absolutely impossible for a comet moving in a
trajectory either parabolic or hyperbolic to become a satellite
of the earth ; 2nd. That for a comet whose orbit is elliptic to
become a satellite of the earth it would be necessary that when
it entered into the sphere of the earth's attraction, its relative
motion, that is to say, the difference between its velocity and
that of the earth, should be only 2,176 feet per second. But
is it possible that a comet whose orbit, 'although elliptic, ap-
proaches, nevertheless, very nearly to a parabola should have
a velocity relative to the earth of only 2,176 feet per second,
whilst it is demonstrated that the relative velocity of a parabolic
comet placed at the same distance, that is to say, at the distance
of the earth from the sun, must, under the most unfavourable
circumstances, be 39,000 feet per second? Besides,' adds Pingre",
' even if this were so, the comet, when transformed into our
moon, would pass in each of its revolutions to the extremity of
the sphere of the earth's attraction, and the least force would
suffice to detach it from us. . . It would then recommence its
orbit about the sun.'
These reasons are derived from the laws of cometary and
planetary motion, and from the principle of gravitation, of
which the laws are the expression ; but it is also evident that
in physical constitution nothing can be more unlike a comet
than the moon. Everything leads us to believe that our satel-
lite is entirely, at its surface at least, reduced to a solid con-
dition. If it has an atmosphere it is the least vaporous possible
629 / M M
THE WORLD OF COMETS.
and is of extremely slight density. Now, all known comets,
those at least which have been subjected to telescopic scrutiny,
appear to have been characterised by a predominance of ne-
bulous atmosphere about the nucleus. The savants of the
eighteenth century, who regarded comets as planetary globes,
nevertheless recognised the entire absence of analogy between
the physical constitution of the moon and that of a comet.
Maupertuis, in order to explain how our satellite, an ancient
comet disguised, might have lost its coma and tail, had only
to invent a new hypothesis, to the effect that some other comet
might in its passage have swept away the atmosphere of the
moon. If I remember rightly, it is not Maupertuis who makes
this new supposition, but some other author whose name has
escaped me.
530
TABLE I.
ELLIPTIC ELEMENTS OF THE RECOGNISED PERIODICAL
THE SOLAR SYSTEM.
COMETS OF
No.
Name of
Comet
Sidereal
Revolu-
tion
Semi-Major
Axis
Perihelion
Distance
Aphelion
Distance
Day of Perihe-
lion Passage
1
Encke . . .
Years
3-285
2-209701
0-332875
4-08C528
1871 Dec. 29
2
Brorsen . .
5-483
3-109618
0-596762
5-622475
1868 Apr. 17
3
Winnecke .
5-591
3-149900
0-781538
5-518260
1869 June 30
4
Teinpel . .
5-963
3-291415
1-770548
4-812282
1873 May 9
5
D'Arrest. .
6-567
3-506698
1-280280
5-733117
1870 Sept. 23
1
Biela North
6-587
3-513740
0-860161
6-167319
1852 Sept. 24
6(
Biela South
6-629
3-528733
0-860592
6-196874
1852 Sept. 23
7
Faye. . . .
7-413
3-801849
1-682173
5-921525
1866 Feb. 14
8
Tuttle . . .
13-811
5-75652
1-03011
10-48294
1871 Nov. 80
9
Halley. . .
76-37
18-00008
0-58895
35-41121
1835 Nov. 15
No.
Name of
Comet
Longitude
of Perihelion
Longitude
of Ascending
Node
Inclination
Eccen-
tricity
Direction
of Move-
ment
1
Encke . .
0 1 II
158 12 24
0 / //
334 34 9
0 1 II
13 7 35
0-8493573
D
2
Brorsen .
116 2 3
101 14 6
29 22 39
0-8080916
D
3
Winnecke
275 56 1
113 33 21
10 48 19
0-7518847
D
4
Tempel .
238 1 6
78 43 19
9 45 49
0-4620711
D
5
D'Arrest .
318 40 5 0
146 25 23
15 39 12
06349044
D
,
Biela. . .
109 20 24
246 5 16
12 33 25
0-7552007
D
61
Biela. . .
109 13 21
246 9 11
12 33 47
0-7561187
D
7
Faye . . .
50 0 27
209 45 28
11 22 6
0-5575383
D
8
Tuttle . .
116 4 36
269 17 12
54 17 0
0-8210540
D
9
Halley . .
304 58 41
55 38 3
17 44 45
0-9672807
R
531
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THE WORLD OF COMETS.
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54-1
NOTE
ON THE DESIGNATION OF COMETS, AND ON THE
CATALOGUE OF COMETS.
Br THE EDITOK. i
The numbers of the comets mentioned in the text of this work (aa ex. gr. in
the tables on pp. 143 and 145) do not always agree with the numbers given in
the preceding catalogue. Thus, Comet II., 1852, appears in the catalogue a8
Comet IV., 1852. This arises from the fact that the comets in the catalogue are
numbered in the order of date of their perihelion passages, while the numbers
in the text often refer to the order of discovery. There is a good deal of un-
certainty with regard to the numbering of comets, in consequence of this double
system, which, however, seems to have been unavoidable. In the 78th volume
of the leading astronomical journal, the Astronomische Nachrichten, No. 1,868
(1872), the editor (Dr. Peters), then director of the Observatory at Altona, and
now of that at Kiel, refers to the designation of comets, and states that the system
of denoting them by the year and the Roman number was first introduced in the
general index to the first twenty volumes of the Nachrichten, and that the numbers
referred to the order of discovery. In the index to the 26th volume the comets
in the year were first numbered according to the order of date of their, perihelion
passages, and generally this arrangement has been followed since ; but, Dr.
Peters observes, this method is inconvenient, as the designation of comets whose
perihelion passages are near together are liable to be changed upon each
recalculation of the elements, and as comets whose elements have not been
calculated thus receive no numbers ; he, therefore, announced that in future
he should number the comets in each year according to their dates of discovery.
But in Nos. 1,871 and 1,872 Dr. Peters prints two letters of Dr. Oppolzer and
Dr. Littrow upon the subject ; and chiefly in consequence of the German Astro-
nomical Society having in 1867 decided that the best numbering of comets was
according to the dates of their perihelion passages, he modified his previous
announcement in so far that he designates the comets in each year a, b, c, . . . in
order of discovery, leaving I., II., III. ... for the order of -the perihelion passages,
and this arrangement has been followed in the Astronomische Nachrichten since
1872. This will serve to explain the existing confusion, which is even greater
than appears, as the numbering in the titles is frequently different to the num-
bering in the indexes ; and the same want of uniformity happens, of course, also
with regard to the designation of comets in other works.
Thus, the comet discovered by Westphul on July 24, 1852, was for some
time known as Comet II., 1852, and would have to be sought for under th's
545 N N
NOTE ON TILE DESIGNATION OF COMETS.
designation in the volumes of the Astronomische Nachrichten about that date ;
but now it would be quoted as Comet IV., 1852. It might have been, perhaps,
better if M. Guillemin had altered all the designations of comets, so as to bring
them into uniformity with the catalogue at the end ; but, practically, the in-
convenience is but trifling, as it is generally very easy to identify the comet
alluded to, and the difficulty such as it is must arise in its full force whenever
there was occasion to refer to the original calculations in the Astronomische
Nachrichten, the Monthly Notices of the Royal Astronomical Society, &c.
It will sometimes be found that the elements used in the text are slightly
different from those in the catalogue ; this is due to the fact, that there are often,
indeed generally, several calculations of a comet's orbit. To take as an example
Westphal's Comet II., 1852: In vol. 35 of the Astronomische Nachrichten there
are three sets of parabolic elements, two by Sonntag and one by Rlimker : and
there are also three sets of elliptic elements, one by Sonntag and two by Marth ;
while in vol. 50 there is a complete discussion of all the observations, with the
elements deduced therefrom, by Westphal. It may happen, therefore, that the
values used are not always identical ; but these slight discrepancies are not of any.
consequence, and I have not thought it necessary to remedy them ; but in any case
where I have found that an error of importance has crept in I have corrected it
(as ex. gr. in the elements of Westphal's comet, which by an accident were
incorrect in the original work).
It must be borne in mind that the catalogue contains only those comets whose
elements have been calculated with some approach to accuracy. Thus, the
comet of 1746, mentioned in the table on p. 145, will not be found in the
catalogue, because only rough elements were computed for this comet by Mr.
Hind; but these were sufficient to lead him to consider it to be identical with
the comet of 1231. Also, in some cases of periodical comets, the eccentricity is
not given in the catalogue. This, of course, happens when only the parabolic
elements have been obtained, and the periodicity has been determined by their
accordance with those of some other comet.
The above remarks refer chiefly to the relation of the catalogue to the desig-
nations of the comets in the body of the work, and it now remains to speak of
the catalogue itself.
The table of the periodical comets (p. 531) is copied from M. Guillemin'a
table, except that I have corrected the elements of Tempel's comet, which by an
obvious accident were erroneous, and have also corrected one or two slight
accidental errors. As for the general catalogue (Table II.) M. Guillemin states
(ch. V., sec. 5) that it is extracted from Mr. James C. Watson's Theoretical
Astronomy (Philadelphia. 1868), and I have reprinted it without alteration,
except that I have added from Mr. Watson's catalogue the name of the calculator
and the hours and minutes of the perihelion passages that were omitted by M.
vjuillemin. The latter addition is of slight consequence, but the former is of some
importance, as, in case it should be desired to 'make further investigations in
regard to any cometary orbit, the .name of the calculator would be of great
assistance. I should state, also, that I have carefully compared Mr. Watson's
546
NOTE ON THE DESIGNATION OF COMETS.
catalogue with M. Guillemin's reprint, and with Mr. Hind's catalogue-, and
so been able to correct several misprints. Thus (except for the introduction ,,r
one comet mentioned below), the general catalogue up to the end of 18GG is due
to Mr. Watson. M. Guillemin has continued it up to nearly the end of 1874 •
but as, owing to the short time that had elapsed since the apparitions of some of
the comets, several of the orbits were only provisional, while several had not
been calculated at all, I have thought it better to complete Mr. Watson's
catalogue myself de novo, so that for the portion subsequent to I860 I am solely
responsible.
This portion which I have formed is the result of a careful study of the
Astronomische Nachrichten and the Monthly Notices of the Koyal Astronomical
Society from 1866 to the end of 1875, and is, I believe, as accurate as it can be
rendered by means of these data. When an orbit has been calculated inde-
pendently by several computers it must, of course, remain a matter of opinion
which set of elements should be preferred, and in my choice I have been
influenced by a variety of circumstances, such as the value of the observations
employed, the length of arc over which they extend, &c. I may observe that
after a comet has been discovered a rough parabolic orbit is frequently obtained
at once from the first three available observations, merely for the purpose of
ascertaining the general path of the comet, &c. This orbit is soon superseded
as the observations are multiplied, and it may appear ultimately that no parabolic
orbit will satisfactorily represent the comet's motion, in which case elliptic or
hyperbolic elements will have to be calculated. When the comet has left us
some time, and all the observations of it have been published, the work of
calculating the definitive elements of the orbit (in which all the available
observations are taken into account) is usually undertaken by some astronomer,
and these, of course, are to be preferred when they exist. Thus, between the
different sets of elements there may be very wide discrepancies, and, if the
definitive elements have not been calculated, it is sometimes difficult to decide
which has the greatest probability of accuracy. Also, in regard to the periodical
comets, the orbits calculated are of two kinds, viz. tho.«e obtained from some
previous apparition by calculating the perturbations to which the comet has been
subjected in the interval, and those obtained from observations of the comet
during the apparition in question. In my portion of the catalogue (and, I pre-
sume, in the portion previous to 1867 also) the elements are sometimes of one
class and sometimes of the other ; in fact, I have merely chosen the orbit which
seemed to me to be most likely to be the nearest to the truth. It is for this
reason that I have thought it proper not to alter the elements given by M.
Guillemin for the periodical coniete in Table I., although it will be seen that I
have sometimes preferred slightly different elements. The elements for the four
apparitions of Encke's Comet in 1865, 1868, 1871, and 1875 were deduced from
the elements given in the able discussion of the motion of this comet by Prof.
Von Asten in No. 2,038 of the Astronomische Naclirichten (1875) . The insertion
of Encke's Comet, 1865, is the only change that has been made in Mr. Watson's
547
NOTE ON THE DESIGNATION,, OF COMETS.
cata.logue. At its apparition in 1865' the comet was only observed in the
southern hemisphere.
The great comet of 1874, discovered by Coggia on April 17 of that year,
and which has been so often referred to in this book as Comet III., 1874,
appears in the catalogue as Comet IV. ; this is because another comet, dis-
covered on August 19, also by Coggia, passed its perihelion on July 5 — three
days earlier than the great comet — according to Schulhof 's elements (Ast. Nach.,
vol. 84, p. 262), which have, been adopted in the catalogue. But the comet
discovered on August 19 was very faint, and but few observations were made of
it, so that there is much imcertainty with regard to its orbit. According to
the elements of Holetschek (Ast. Nosh., vol. 84, p. 269), the perihelion passage
took place on July 19, eleven days after that of the great comet. It is, there-
fore, doubtful which comet first passed its perihelion.
What has been said will show the nature of the uncertainties .attending
cometary orbits; and I need scarcely add that for purposes of exact astronomical
research no catalogue, however excellent, can supersede the necessity of referring
to the original calculations and observations. For example, it may happen that
neither an elliptic, parabolic, nor hyperbolic orbit satisfies the whole of the
observations satisfactorily, and in this case the selection of any of the sets of
elements may be all but arbitrary.
In conclusion, I will repeat that the catalogue only contains comets whose
orbits have been calculated ; so that, for example, the comet seen by Mr. Pogson
and referred to in the note on Biela's comet (p. 265) is not included, as, since
only two observations were made of it, no orbit could be calculated, A list of
comets whose orbits have not been calculated will be found in Mr. G. F.
Chambers's Handbook of Descriptive and Practical Astronomy (1861.)
The reader will notice the extraordinary dearth of comets in the last two
years. No other comet except Encke's was seen in 1875, and none have been seen
during the present year. This complete absence of comets, following years so
rich in comets as were 1873 and 1874, is very remarkable.
548
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UNIVERSITY OF TORONTO LIBRARY
Astron Guillemin, Amedee
G The world of comets,
tr. and ed. by James
Glaisher.
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